MyCommerce handle all our download sales. They are based in the USA and laws force them to add any local sales tax for your state. They also have to add VAT to orders from countries in the European union and sales tax to some other countries which have international agreements. Tax will be at the rate set by your own country so the amount is not known until you go to checkout and state your address. At this point you will see the final price and can decide if you want to proceed.
DP Software is in the UK but due its small size no longer charges VAT. For overseas orders, you may get charged any local sales taxes when the package is delivered to you - plus a collection fee added by the delivery service.