I won Olfa mats on eBay for a local non-profit. They were shipped flat.
I think an old clear mat I have is a Salem mat. It's okay, but I prefer the green Olfa mats. The lines markings on the Olfa are much more refined than on the Salem; the unmarked side of the Salem is very rough. I would never mix pieces cut on both mats; I'm sure the cuts would not match because of the difference in line widths.
Are you sure mats would be sent rolled up? I'm pretty sure they're all mailed out flat these days. You could email or call the company before ordering, to find out.