I have copied this from a page at http://www.business.mo.gov/ this is a great plain talk and well organized introduction to how to be legal in MO.
Sales Tax
+ What is subject to sales tax in Missouri?
All sales of tangible personal property are taxable unless there is a specific exemption listed in Section 144.030, RSMo .
All sales of services listed as taxable in Section 144.020, RSMo , including telephone and telegraph services, are subject to sales tax.
+ Should I pay sales tax or vendor’s use tax?
Sales tax is applicable on all sales made from a location within the state of Missouri.
Vendor’s use tax is applicable on all sales made by out-of-state vendors where goods are shipped into Missouri and where title passes within the state of Missouri.
+ When am I required to file my sales tax returns?
Sales tax returns may be filed on a monthly, quarterly or annual basis. Your filing frequency is determined by the amount of state tax (4 percent for regular locations and 1 percent for food locations) due. Local tax is not included when figuring your filing frequency. The filing frequency is determined by the total state tax due on the return as a whole, not by each location.
State taxes collected of $500 or more per month are to be reported on a monthly basis.
State taxes collected above $45 per quarter but less than $500 per month should be filed on a quarterly basis. The quarters are as follows: January through March, April through June, July through September, and October through December.
State taxes collected less than $45 per quarter should be filed on an annual basis.
+ Will my filing frequency ever change?
Your filing frequency is reviewed by the Department of Revenue on an annual basis. If this review indicates that your filing frequency should be changed, the change will be made and notification will be sent to you.
Note: It is very important that you keep your address information current with the department.
+ What are the due dates for filing my returns?
Monthly returns are due on or by the 20th of the following month, except on quarter ending months. For example, your monthly February return is due on or before March 20. The due dates listed on the chart for quarterly returns should be followed when filing quarter ending months such as March, June, September and December.
Quarterly returns are due on or before the last day of the month following the end of the quarter. For example, your return for the January through March period would be due on or before April 30.
Annual vendor’s use tax returns are due on or before January 31 of the next year. Annual consumer’s use tax returns are due on or before April 15 of the next year.
| Monthly Reporting | Quarterly Reporting | Annual Reporting | 2006 Actual Due Date |
|---|---|---|---|
| January | February 21 | ||
| February | March 20 | ||
| March | 1st Quarter | May 1 | |
| April | May 22 | ||
| May | June 20 | ||
| June | 2nd Quarter | July 31 | |
| July | August 21 | ||
| August | September 20 | ||
| September | 3rd Quarter | October 31 | |
| October | November 20 | ||
| November | December 20 | ||
| December | 4th Quarter | Year | January 31 |
Download a copy of our current tax year calendar indicating due dates for all tax types.
+ What if the due date to file a return falls on a Saturday, Sunday or a holiday?
When the due date falls on a Saturday, Sunday or a holiday, your return will be considered timely filed if it is postmarked by the next business day.
+ Is a sales tax return required even if my sales equal zero?
Yes. Every business with a sales tax license is required to file a return even though no sales were made during the period covered by the return.
+ Can negative taxable sales be filed for a business location?
No. Negative taxable sales cannot be filed for a location on the return.
When the credits allowed are greater than the tax collected, an amended return and Application for Refund/Credit ( Form 472B 
) must be filed for the period in which the sales were actually filed.
+ What is the difference between “gross receipts” and “taxable sales” on my sales tax return?
Gross receipts equals the total amount of sales your business had for the period in which you are filing the return.
Taxable sales equal the total amount of sales your business had for the period in which you are filing the return plus/minus any sales on which you did not collect sales tax. These are claimed in the adjustments column of your return.
Note: Your taxable sales should always equal your gross receipts plus/minus any adjustments.
Every vendor must file a sales tax return showing the amount of taxable sales to his/her customers, as required by law.
+ If I have included my sales tax in my gross receipts, can the tax amount be backed out?
The amount of sales tax collected should not be included in your gross receipts. If the sales tax is included in your gross receipts, it should be backed out. To back this out, take your total amount of gross receipts, including the sales tax, divide by 100% plus your current tax rate.
Note: A record of the adjustment claimed on each return must be maintained in your files. The Department of Revenue will review this information if you are audited.
+ How is the two percent timely payment allowance calculated?
On all sales tax returns filed and paid by the required due date, you are granted a 2 percent timely payment allowance. Take the amount of tax due times 2 percent. Then subtract this amount from the amount of tax due.
+ What determines if a return is timely?
Your sales tax return is considered timely if it is postmarked on or before the required due date. If a metered postmark differs from the U.S. Postal Service postmark, the U.S. Postal Service postmark will be used as evidence of timely filing.
