I use flat rate postage, and find it works great (most of the time) for my items. I charge a low letter rate and sometimes need to use parcels so I can lose a little on occasion (on postage anyway), becaue I don't exactly subsidise postage costs in my item prices, but there are still savings for me if buyers purchase multiples (resources, like packaging and labeling, PayPal fees if one payment was made etc).

If you use the new Sales Maximizer thing and set up postage discounts / promos with it, the banner on listings prompts buyers to use the cart. (I have still had people paying separately, though, apparently it's not visible on apps).

There is one other alternative, and that's a postage cap - that's what I used when I sold clothing. 1 item was normal postage costs, but people couyld buy any number of items and pay a maximum of $12 postage (based on what was then the 5kg express satchel, which was a bit more than that at the time). That way, no one is ever paying more than they need to for a single item, and people who buy multiples have an incentive / reward.

If anyone wants to know more info about how I use a flat rate postage etc, I'll be happy to share a little later tonight (errands to run, birthday shopping to do :D)