I have used Sportlots for a long time now. Almost 20 years since I got back into the hobby in 2006. It has changed a lot over the years, but there are always tricks to keep costs down. Here are a few that I use.
1. Check postage rates of a seller before adding to your cart. You are less likely to just "go ahead and get it" if you check pricing beforehand. Saving buyer's remorse really helps the wallet.
2. When I shop, I focus a single card or set for that order. I'll add the best priced card I need from that set to my cart and then when I am finished, I go to the cart and make price cuts. I always cut until I get down to three of fewer sellers per order.
3. When you get down to your small number of sellers, ALWAYS add cards up to the shipping price change. Sportlots seller change shipping prices at tiers of 1, 4, 10, 16, 25, etc. For example, if you have 2 cards you want from a seller, go and look for another 2 cards from that same seller because the shipping price will not change. It also helps to sometimes go to the tier to bring down the average price per card.
4. Compare box shipping to budget shipping. If the box shipping is 50% cheaper than the budget, get it. Yes, you have to pay later for shipping the box, but it almost always ends up cheaper in the long run. Some sellers actually charge no shipping for box orders. If the actual shipping costs are cheaper than what Sportlots charges you, they will credit your account with the overage. I get that refund about a third of the time and it feels like I get free cards.
5. BE PATIENT! If you cannot be patient, then Sportlots is likely not for you. Ebay would be a better, although much more costly, way to get cards for you.
6. There is a key to knowing the types of cards to get on certain sites. Using Sportlots for buying autographs, memorabilia, or serial numbered cards is usually a bad idea. Sellers tend to overprice them on that site. I use COMC or eBay for those cards. The best cards to get on Sportlots are vintage cards, modern base cards, inserts, or non-numbered parallels. You find great deals on them there.
Hope that helps.