Border run to Mae Sai

I have to do a border run soon to start my 2nd entry. I've never been to Mae Sai before, and this will also be my first border run.

I have some questions.

First, do they still accept $10 or must it be 500baht? And will the $10 have to be clean and new and does it have to be a ten spot or can fives work as well?

Second, I want to spend some time in Mae Sai shopping and eating and drinking.... the buses from Mae Sai to Chiang Mai depart too early. Are there later buses from Mae Sai going to Chiang Rai where I can then board a bus to Chiang Mai?

Thanks!