It is pretty normal to enter those numbers. If you don't feel comfortable using a credit card you can use Paypal as well.
Unfortunately we won't be porting this version to Steam.
Ugalek, Amazon uses the 3 digit security code on your credit card, eBay does, and every site I paid that requires credit card requires the digit security card. That way, people won't just be able to use your credit card without the security code. Also, Itch.io is safe and I never had any problems. Try using Paypal if you want as that is what I did and it worked fine.