Playa del Carmen brides could visit Sam's in Playacar (or Walmart for things that are not in "bulk"). Easy to find, here is a map:
Costco in Cancun is a fabulous place, easy to find (about 1/2 hour from anywhere in the hotel zone of Cancun, about an hour from Playa del Carmen). Totally safe (to address a previous poster). A fun addition to your bags would be some specifically Mexican candy, there is a whole aisle full of treats at Costco! I'm a big fan of "Swinkles" (like sour candy with a little spice), the kids here love "Pelo Rico", popular chocolate is "Carlos V" (just milk chocolate) and "Bubu Lubu" (chocolate with marshmallow). Just tossing out some ideas!
Here's a map to Costco