How can a company that did not charge sales tax figure it out at tax time?

Probably at that time the simple and easiest way possible gross sales X the sales and use tax rate at that time and JUST pay the amount owed to the local taxing authority as required to do since the amount is NOW past due RIGHT. 1000.00 X 10% = 100.00 sales and use tax owed at that time OK Hope that you find the above enclosed information useful. 06/17/2013

if the sales tax is included on all their taxable sales take the total amount of the sales and divide by 1.xx % of the sales tax rate, this gives you the taxable sales and the difference from the gross and this amount is the amount or your sales tax

If tax was never added to the sales you would multiply sales x tax rate in your state. For example, if yearly sales were $50,000. X 6% = $3,000. Tax due.

They would owe the tax on top of the sales. Sales x sales tax. Built in sales tax. If the rates were inclusive of tax then it is sales(1-tax rate). Simple algebra. Probably must be paid to taxing authority quarterly.