Many of the towns on the Cape will let you buy your beach sticker online. Here is some information for a few of the Cape Cod towns.
Bourne https://www.townofbourne.com/natural-resources/pages/beaches-boat-ramps-parking
Sandwich https://www.sandwichmass.org/168/Beaches
Falmouth http://www.falmouthmass.us/152/Beach-Parking-Stickers
Mashpee https://www.mashpeema.gov/town-clerk/faq/how-do-i-get-recreational-beach-sticker
Barnstable https://tobweb.town.barnstable.ma.us/Departments/Recreation/
Dennis https://www.town.dennis.ma.us/sticker-sales/pages/sticker-fee-schedule
Yarmouth https://www.yarmouth.ma.us/652/Beaches
We hope you have a fun summer on Cape Cod!