I'm curious to know if this has happened to anyone else. I got standard ($3) shipping on my shirt. It came today and, low and behold, it was inadequate postage. I had to leave money for the flippin mailman because the money I paid them apparently didn't cover the full shipping cost. Horribly unprofessional, IMHO.
I received mine today no problem. I didn't look at the package but I'm assuming that they shipped it via First Class. First Class covers up to 13 ounces for shipping unless they have a handling fee as well. If it weighs more than 13 ounces, it has to be shipped via Priority Mail or some other service.
How much did the post office say you owed for shipping? I always thought the post office would send the item back to the original shipper and tell them invalid postage ... I've never heard of a package recipient being charged more for the cost of an item's shipping if the shipper didn't pay enough. You must have a really strict post office!
You should contact RIPT instead of saying that's "horribly unprofessional". Could have just been an honest mistake when weighing your package, and we're only talking about 20 to 40 cents max for one shirt, unless you ordered multiple shirts and it was still shipped at the 1 shirt weight.