How you can add a sales tax on bill you have to pay?

The sales tax rate should be applied to the sales value.
The resulting sales tax amount should be separately disclosed in the invoice.

Sales Tax is an indirect form of taxation which is levied on all applicable sales transaction based on law of land which is normally recovered from the purchaser of the goods.
Sales Tax is computed on the final sale price & its normally added below line to sales invoice to know the money due or needs to be collected from the purchaser.

to find sales tax you should get the final final selling price * sales tax %

Value of goods with out Sales tax =Value of invoice or bill divided by (1+sales tax rate in %).
in this way you calculate value of good with out sales tax then deduct this answer from the bill amount then answer is sales tax amount .

Using the  settings for the program is only once and determine the tax rate and the agencyresponsible for the debts and then be automatically calculatedon the establishment of an invoice sale ...من البريفيرنس  ونختار ضريبه المبيعات يمكنك تحديد نسبه الضريبه والجهه المسئوله عن التحصيل وتحسب تلقائيا عن انشاء اي فاتوره بيع

1- EDIT

2- PREFERENCES

3- SALES TAX

4- COMPANY PREFRENCES

MARK YES ON (DO YOU CHARGE SALES TAX)

cost plus cost in to sale tax rate