Prop Money is running three overlapping percentage discounts across the store right now. The 15% off deal is the main offer on the page and represents the best baseline saving for most shoppers, though it's worth comparing the alternatives depending on what you're buying.
Buying a regular selection of full price items should be straightforward: apply the 15% off discount to your entire order. This saves you 15 cents on every dollar you spend, so a CA$100 basket drops to CA$85.
If you're shopping the sale section, the up to 75% off deal on sale products might deliver a bigger saving than the 15% off blanket discount. Sale items can already have steep reductions applied, and stacking an extra 15% on top of an already discounted price may deliver less than buying items that are marked down 75% from the start. Calculate the final price both ways before you checkout to confirm which comes out ahead in your basket.
Smaller shopping trips under CA$50 still benefit from the 15% discount, though you might also want to check whether the 10% off option applies—it could be a stepping stone offer with a lower minimum spend, though the page doesn't specify thresholds. A CA$40 order with 15% off lands at CA$34, which is solid savings for a quick purchase.
Larger baskets over CA$100 should definitely stick with 15% off. On a CA$150 order, that's CA$22.50 in savings, which outpaces smaller percentage discounts by a meaningful amount.
Only one discount can be applied to your order at checkout, so compare the final price your basket reaches under each offer before you pay. The offer that produces the lowest total cost is the one to take.