I tried the same, and I think their shop system is bugged.
Using an Austrian IP I get:
WinRAR € 29,95
19% VAT € 5,69
Total € 35,64 ($ 50.38)
Using an IP from USA I get (Country Austria selected)
WinRAR € 21,83 ($ 29)
19% VAT € 4,15 ($ 5.51)
Total € 25,98 ($ 34.51)
So, a different IP saves you 10 Euros? Stupid.
Guess best is to ask someone in USA to order it and you pay this person via Paypal. Then it costs € 21,83 ($29) only. I'm not sure charging VAT is okay here, since they're not shipping anything. If you want a different IP, try CyberGhost VPN (coupon code "quale157" gives you one year for free (untested))
Edit: Attached two screenshots
Edit 2: Asked Winrar about it... here's the answer:
We have different pricing strategy for different markets. Such price
differences are very common in daily business (e.g. the computer
equipment or jeans that are cheaper in US, the digital camera bought in
Asia...). That's the way things are. Markets differ and so do prices.
Well, I really like WinRAR, but this sound stupid. It's a software licence sent by email, no camera or jeans. Oh well, I'm glad I registered it years ago.