I would love to put up a notice above all our listings about combined shipping, but our five stores are so different that I can't come up with anything except a bland "we combine multiple puchases." Which isn't completely accurate, because sometimes we can't combine items.
For our coin, stamp, and picture shops, we can give the exact amount each additional item would cost. But . . . the amount is different for each shop. When I tried a long notice, including the amount for each, it looked ridiculous and I doubt if anybody would read it.
Also would need different wording for the bookstore because books are so heavy. This would need to be directed to international buyers, where combined shipping might have to go by Priority Mail and so would cost more than individual shipping by International First Class.
Our general store needs a couple of warnings, such as "We can't ship an LP and a stein in the same box."
I realize this is low-priority, but if you ever have a chance to add it (perhaps when you're doing something else in that part of the system) it would be very useful.