CalamityJanet
Ahh, the ultimate in stealthy bills. Use invisible ink of course.
Actually, its UV ink. It shows up under "black" light and man does it glow. Most chain convenience stores (Mac's, 7-11…) use them, plus some bars on the dance floor or if you go "glow" bowling.
I had a regular self inking stamp made, then instead of putting regular stamp ink into it I used the UV ink. $5.00 for a 2 oz. bottle at Rose's. They weren't sure if it would eat the pad or not. I've had it since just after the last gathering and no problems yet. The only thing I found was that the ink has a very high alcohol concentration and dry's out fast (over the weekend) so, the pad comes with a cover and I just pop it out and cover it whenever its not in use. Only had to fill it once since I dried it out.
The other toy I got was a hand held UV light ($20 at Staples, right next to the $50 plug in one's the convenience stores use) Cuz you can't tell if you've stamped unless you shine the light. Ya, that's why I needed it ;-)
ps. Cats don't glow, but sock lint sure does