How important are possibilities for accepting phone/fax/mail orders and International credit cards for running uISV internationaly?

It depends slightly on the market for your product (individuals vs small business vs big business, etc) but in general simply supporting credit cards should be fine (in fact, I wouldn't even worry that much about amex/discover - visa & mc cover the bulk of the market).

Over a 5 year period, I would say about 1-2% of my orders (for a C++ development library) were from a method other than credit cards. Most of those were not by phone or fax, but rather by wire transfer and almost all were from BRIC countries.

I would suggest going with whatever provider is easiest for you to set up (or whichever takes the lowest commission). If you get email from customers complaining that you don't take phone or fax orders it's easy enough to migrate to a provider who will do that.

Also, don't neglect the fact that, by default, you kind of already do take phone/fax orders. Just take them yourself using a US based Vonage phone number or something similar.

For a cheap little program like yours, selling through a webshop that accepts major credit cards (and maybe paypal as well) should be all that it takes. Unless your program is to big for downloading, you (really!) want electronic delivery, to avoid the costs and hassle for shipping CDs etc. Phone orders and fax orders obviously don't play well with electronic delivery.