What is the ruling on programming a web page to sell products that includes calculating and adding sales tax to the price, and is the programmer considered to be in the position of the seller, bearing no sin for this?

The forbidden type of taxes is what is known as mukūs (illegal taxes/customs), which are taken without a legitimate Shar'i right. The one who collects mukūs will not enter Paradise. Based on this, designing the page does not become sinful unless the taxes included in the design are of the forbidden type. These taxes cannot be categorized without knowing the reason for which they are collected.