I got my STD magnets from Save the Date Magnets and I loved them! They sent envelopes with the magnets and they did NOT require extra postage. 2.65" X 4" GREAT SIZE, great price and quality.
I decided to go with a letter to add to the envelope to give people info about the travel agent. But if i remember correctly, I had to add a 1 cent stamp to them b/c of weight, not bad at all.
Good Luck!