I live in Europe and book quite a few cruises for us -- mostly through U.S. agents, because the cruises are much cheaper that way. The problem is generally not about where the CC is issued, but that you need to have a U.S. mailing address (I do, through a very close relative living in the U.S.) connected with your reservation. Princess is one of the strictest cruiselines about this but, even so, keep checking until you find an agent who can (is willing to) use their address for your booking. As someone has already said, it may not be worth a lot of aggravation to save the $200. Be careful if you choose to book with an OTA in the U.S. (as opposed to a "bricks and mortar" agency). A couple of the big online TAs have gone bankrupt in the past couple of years and, unlike booking in the U.K. or Germany, you are not insured if the agency goes "belly-up", especially if your payments went to the agency's account and not directly to the cruise line.

BTW -- I don't think that any of the "standard" cruise lines use anything other than electronic documents anymore, so not much worry about mail going astray (other than sales promotions after you have cruised with the line). You can request printed (as in booklet form) docs, but you may have to pay an extra fee for receiving them that way.