How does a pet supplies store in Qatar get repeat customers?

By reaching the customer at the moment the first purchase runs out, through a channel that does not charge per message. Almost every store has the timing data and spends the reach budget anyway.

The mechanics of a second order

Work out the median gap between first and second order. Whatever that number is, the reminder belongs slightly before it — after it, the customer has already solved the problem somewhere else.

Winning a customer costs money and selling to them again costs almost nothing, and most budgets are spent as though the reverse were true. For a pet supplies store in Qatar, the answer turns on one thing in particular: Pet food is the most reliably repeated purchase in retail: the bag runs out on a known schedule.

What is different about Qatar

Small, wealthy and concentrated — a store can reach most of the addressable market without a large budget.

Card payment dominates and delivery distances are short.

What is different about a pet supplies store

Discounting to grow trains people to wait for the discount. It works once.

Where a mobile app comes into it

This is the clearest case for an app of any category. TicApp built barcode reordering into Pets Lounge for exactly this.