Guinness USA certainly capitalises on St Patrick's Day over here -- they've shown quite a few ads on TV here equating St Patrick's to Christmas, with everyone getting cases and kegs of Guinness and Harp....
[Nik] If St George had been asked what his birthday was, would he reply, "Nik's birthday"? (NB Nik's email address removed by request -- rab)
[ZK] I'd help you with the American marketing of St David's Day, but most Americans wouldn't know a Welshman if he came up and shoved a leek up their collective arses. ;) See also Church, Charlotte, meeting George W. Bush
[HB] Nowt Welsh at all, I'm sorry to say. Although I have tried and dislike laver bread, I would have bought any (or all) of the other items that you mentioned. I would also like to mention that difficulty my Irish father-in-law suffers in trying to find me a St David's Day card, on the back of my always sending him a St Patrick's Day card. Sceptics, note - he managed it once, so such cards do exist!
[ZK] You need to get in touch with this fine body of people. Who will no doubt explain that it was, of course, the Welsh who discovered America in the first place, a fact disputed by Native Americans who unaccountably reckon that they got there first!