Hi Susano,
Pretty much all of the grocery stores will be open on Sunday afternoon.
I'm also going to do a blatant plug here, if you would rather have your groceries pre-ordered and pre-delivered to your vacation property prior to your arrival check out Lagniappe Provisioning at
www.lagniappe-belize.com (yes, it's my company, hence the blatant plug).
Cheers,
Simon...