The free shipment thing is a unique ability of Bulgaria. It does only affect a few cards though; without this ability the civ would be very UP. And only from year IV onwards they can get a lot of troops from cards; but it requires you to get there first (not that easy, as things get more difficult if you get raided). The unique is the only way to make this civ work (but it has other problems to worry about though)
Are you playing on non-expert difficulties? Maybe the shipment generation is a bit too high for the slower pace compared to expert level. IDK.