Manufacturer coupons are treated as cash, same as Shoppers. So if you spend $50 and use $20 in coupons, so pay $30 you'll still get the $50 spend promo ie today in your example with P&G, 250 airmiles.
Exception, survey $5 coupon, you must spend $25. So if you spend $50 and have $40 worth of coupons, the survey coupon will not work because the subtotal would only be $10.
Note that In-store Rexall stickers on products, reduce Rexall's price, so you need to spend more. Again, same as what Shoppers currently does.