Compare a basket you would normally buy
Choose several recurring items and record the purchase size, brand if important, and date. Compare the same kind of coffee and the same amount of rice at each shop. A discount on a product you rarely buy should not determine which store you call the cheaper weekly option.
Separate the basket price from the trip cost
Record the actual prices and date on one trip at each shop. Add the cost of an extra journey and the time it takes. When the cheaper basket requires two buses or a special stop, one slightly higher bill at the usual shop can still make more sense.
Suppose the usual basket is $28 nearby and $25 at another shop. If reaching the second shop requires a $4 fare you would not otherwise pay, the apparent $3 saving disappears. If the shop sits on a route you already take, the calculation changes. Use the actual extra cost of that trip.
Repeat when a price looks unusual
A promotion can make one week's result unrepresentative. Note the sale and check the normal price on another visit before changing your routine. If each shop wins on different items, compare the benefit with the time and effort of making two stops. The lowest combined shelf price may not be the best practical household shop.