Why not do something creative like have the investor sign a dollar bill (of any denomination) and give it to the founders? Every founder comes prepared with some cash in their wallet, and then when you confirm a deal the founders ask the investors to sign the dollar bill with a Sharpie/pen. On it would be some sort of short-hand for the deal valuation... Cash is more ubiquitous than phones - even impromptu, it's high…
Numerous Reasons: o Everyone has a mobile device at all times, trying to remember to carry cash and/pens is an extra step. Cash, in the valley, is not as ubiquitous as a phone. In fact, it's not as ubiquitous as a smart phone. o Texting is instantaneous. In fact, when we're sitting at a table having a conversation, it's not unusual for some people to be texting each other instead of talking to avoid creating a break…
So whether that's scrawled on a dollar bill, transmitted officially via the "PG Handshake Protocol", or signed on your ass with a picture in the mirror for posterity... doesn't matter, have fun with it and get it done!