Regular customers write their names, addresses and phone numbers on the back of their store (fiscal) receipts, and mail the receipts in to the VAT office.
All the fiscal receipts which the VAT office receives before the cut off date - which is BEFORE the 13th of the month for all receipts dated the previous month - are put into a big barrel and a drawing takes place.
About ten winners are selected each month.
The winners receive the equivalent of ten times the amount of VAT they paid on the receipt that was drawn. Some winners have gotten prizes of up to the tens of thousands of Euros. (I strongly suspect these are contractors.) The minimum prize amount is 233 Euros for receipts with VAT totaling less than 50 cents.
(Photo linked from the Times of Malta online edition.)