You can use Paypal, or a prepaid debit card should work just as well as long as it's one of the major brands (Visa, Mastercard, etc.. assuming they have those in Europe?).
If all else fails, you can send an email to firstname.lastname@example.org and ask for payment options, they will probably refer you to this order form: https://www.stardock.com/order.txt
But I'd advise talking to them first if you have to go that route just to make sure that's how to do it.
Paypal: I recognize the option is there, it's nice in theory, but I will probably never trust that company after all I heard about it.
Visa, Mastercard, etc: Would be the last option, I'd have to get it first (Meh, will cost more then the game )
The form does not include entrenchment?
So I take it, there is not other way to buy entrenchment other than Impulse? And now other way planned?