I've gotten ahead of myself. Sure, you need to be able to seal your letters, but you have to write them first, don't you? So you better pick out a good ink. One that can take anything the Postal Service may throw at it.
While no ink can survive inconsistently applied non-machinable charges, Noodler's Black can, I'm told, handle most anything else.
It's a dark, saturated black that bonds to the cellulose in paper (that's what they keep telling me, anyway) so it sticks for good. And for you patriotic types, it is all made in the USA--the glass, the cap, the ink, all of it.
Buy through iSellPens.com for $12.50 for a three-ounce bottle.
Image via the same.
I remain, as ever, your most humble servant,