There's a joke among Belizeans that although Mexicans believe that Chetumal is the southernmost city in Mexico, it's actually the northernmost city in Belize.
While there are no shopping malls in Belize, the city of Chetumal, Mexico is an easy day trip from Belize. Chetumal has a large modern, indoor mall, several big box stores, auto parts, office supplies, movie theatres and a wide range of restaurants.
On any given weekend, the number of Belizean license plates in the mall parking lot rivals the number of Mexican plates.
There is frequent and inexpensive bus service that runs between Corozal (the city in Belize closest to Mexico) and Chetumal, so it's not necessary to have a car or take it across the border. Taxis in Chetumal are ubiquitous and cheap.
From San Pedro, there is water taxi service direct to Chetumal once a day. The boat arrives in Chetumal about 10 am and departs at 3:00 pm.