Tori, I don't know the weights of what you sell but I can tweak my weights to where I am not overcharging on multiples and still have enough for single items on lightweight and heavier items both.
On my male dog diapers.
4 of my sizes weigh approx. 1.2 to 1.8 with a mailer.
If I put 2 oz for the weight, on multiple orders to much is calculated on the smaller sizes.
So what I do is put 1.5 oz to 1.75 oz. depending what size.
This has worked extremely well, for 1 item, the shipping is still based on 2 oz.
But for multi orders, the shipping costs are very accurate, most times exactly what it should be, no undercharging or overcharging way too much. It might figure up 25-30 cents to much s/h on 6-7 diapers but I use this little extra for supplies.
Same on my goat coats, I have to tweak the weights slightly to get multiples to work.
2 sizes I sell weigh approx. 1 lb. 10 oz with a box.
Problem is if someone buys 2 it calculates at a 3 lb. 4 oz rate overcharges shipping.
I can get 2 coats in a slightly bigger box for 2 lb. 12-14 oz.
So what I do for these sizes is I only put 1 lb. 6 to 7 oz.
When 2 are bought it calculates at the 2 lb 12-14 oz rate.
I have tested this adding, 3, 4 and even 5 to my cart.
And the shipping is exactly right everytime.
I also do this on other items, especially on items weighing between 1 lb. 1 oz and 2 lbs. you can often tweak that weight to where it will calculate multipes ok.
This is no different than CS on eBay, the box, item and packing are ALL figured in on that listing for shipping cost.
I've tweaked weights on my listings there for years to get it to work out for multiple orders.