This worked well for DD back in 2015:
When we learned DD mailing address for PS, which I think was just before we left for IDay, we sent of a manila envelope to friends and team mates DD said she'd like to hear from during PS.
Inside was a short note explaining that: DD was going to be stressed/tested during Plebe summer; that she wouldn't have a phone/email/text available to her; and that the recipient was someone important to DD that she'd love to hear from over the summer. We also included 3 stamped envelopes with DD's plebe summer address on each.
I think we sprung for postage on 3O envelopes. DD didn't count her mail, but said she got around 20 much appreciated notes from her friends this way. I doubt she would have gotten 5 letters if her e-mail/texting friends had to address and stamp an envelope.
Best to all Plebe-to-be parents.