is it a sin?

I haven't been marking all the bills, am I going straight to hell for that?

No it's not a sin, just the next think to it. So you won't got to hell, just next door…..

indeed, your address will be 662… the "neighbour of the beast"

you get credit for the entry (a mere pittance of points… if you DO actually enter it) and you might conceivably get a "stealth hit" from the note, but, no, there is nothing wrong with what you have done – i have had to do it at times (for a variety of reasons) as have many others here.