Sales tax can make calculating the final cost of the invoice slightly confusing in some cases. If the buyer and seller are in the same jurisdiction, you can simply include the sales tax for your jurisdiction. In many cases, you may be required to collect the tax as the seller.
However, if the sale is taking place between jurisdictions, the sales tax is normally assessed whether the buyer took possession. In some cases, the buyer may be required to self-assess taxes. In other cases, you may need to do it. Seek the advice of a tax expert to learn your responsibilities.