do recs have to be mailed?

Every sorority woman either knows or can find out how to submit recs. It's not up to the pnm.

in my experience, they all have to be mailed.

For my organization it's done online and only members or alumna can write them for pnm's. You should not have to mail in anything.

Just send them/ give them a pre stamped envelope with the address given on the Panhellenic website for their house that they are writing the rec. Many do it all online ( mine does and also mine allows for current members to write recs for girls, just as long as it is not for their chapter). Some allow for it to be done online or mailed with the form they get in their magazine every year. Whatever is more convenient for that member. If you are really concerned, you can go to each national website for each sorority and find out. Many alumna will let girls know they do it online and to not bother with the stamped envelope and that they need the PDF version of their headshot.