Do you provide physical mockups?

We'd rather not.

Before computers and the internet, proofing was done by printing the art on a piece of paper and taping it to a bag. Ah, those were the days. There are several problems with proofing this way - it's far less efficient than emailing a digital proof, costs more in terms of time and money, cannot be depended on for color or print quality accuracy (since it is a completely different type of printing), and doesn't really provide any benefits over a digital proof.