It used to be the larger the pack the better the price, but that's not always so these days. For example, a 50 cent coupon, doubled on any size of soap detergent could make the smallest size the most economical in terms of least cash spent. Many times you may get a box of a less expensive detergent, such as the store's brand name or a generic equivalent for close to nothing, because of the doubled coupon savings. Avoid buying plastic bags. Most stores give these away free.
Plastic bags are available in the produce and meat sections.
Separately bag each item and save them for re-use. When asked if you want either a paper or plastic bag ask for paper inside of a plastic bag and you have a ample supply of ready made garbage bags. Avoid buying cleaning aids, cleansers, etc. They are very costly and prices vary greatly with the brands. The best cleanser in the kitchen is simple soap powder in ammonia.