Masymas is a local grocery chain of some 120 stores that recently celebrated its 30 years anniversary.
The store that visited recently is close to the beach and the marina of Altea. Clean hall, wide choice of food and drinks, reasonable prices (especially considering their seafront location).
You need a Masymas if you are renting an apartment instead of staying in a hotel, which is very common. The store was quite busy when I visited - people were loading up on wine and jamón after the day spent on the beach.
I guess they deliver to homes, there was a delivery van parked outside. You can sign up for their loyalty program in-store and get some discounts.
The company maintains a lifestyle blog (only in Spanish): http://www.vidamediterranea.es
Conclusion: convenient for holiday-makers and locals, reasonable prices. Also love their name ("MoreAndMore") and design of stores and vehicles :)
UPDATE: just found them on twitter, they publish daily ads like this one, cool: https://twitter.com/masymas_fornes/status/761820360392204288?s=09 read more