Roshan

@R C-R, @Patrick Connor, Thank you for your explanation. I'm very sorry I forgot to post what happened after I contact Apple. Honestly it's my fault. Somebody, probably my sister downloaded some apps from my iPhone (2 small apps, games). Since I didn't get the detailed purchase receipt I didn't know how the total were made until I contacted Apple. So they told me there were 2 additional purchases apart from the Affinity Design purchase. So now all are fine and I got the detailed receipt also.

Apple is required by law to collect sales taxes on retail purchases made in locations subject to various U.S. taxing authorities. This can include state, city, county, "special purpose district," and local or regional transit authorities.
 
Every state is different. For example, in Texas the state sales tax rate is 6.25%, but the other taxing authorities are permitted to add up to an additional 2%, for a maximum of 8.25%. This is based on the 'point of purchase.' For online purchases it is the billing address; otherwise it is the physical location of the seller.
 
$54.97 on a $49.99 purchase works out to slightly less than 10% total tax. This suggests the OP's billing address is in one of the highest U.S. tax rate locations, probably Tennessee, Arkansas Alabama, Louisiana, or Washington State.

Roshan,
 
Welcome to the Serif Affinity Forums.
 
Apple sometimes has to pay local taxes that can apply to App store purchases, I expect that is what has happened. Did you get to the bottom of this?

For every U.S. location where Apple has a "physical presence" (like a bricks & mortar Apple Store) Apple is legally required to collect applicable state & local sales taxes for all purchases, even for online software purchases. This is based on your billing address, which you can check by clicking on the "Account" link in the Mac App Store in the "Quick Links" section.
 
Normally, you will receive an email from "iTunes Store" or similar with an itemized receipt for your purchases, which will show the taxes as a line item after the subtotal. You can also check recent purchases using the iTunes app, as explained here.

App store payments are solely controlled by Apple. Nothing to do with Serif.
 
Contact Apple (there will be a link in your membership details somewhere) and they can help.