If a customer pays for merchandise with a credit card, tax applies in the same way it applies to other sales. You must collect and pay tax on the full selling price of the merchandise, regardless of any fees you may owe credit card companies (WAC 458-20-108).
Starting October 1: Some business services are now subject to retail sales tax, as required by state law, ESSB 5814. When you buy these services, vendors should add sales tax to your bill. If you sell these services, you should begin collecting retail sales tax. Learn more about Services newly subject to retail sales tax.