Have you picked a difficult subject. I've struggled with the combined shipping for years. With the new DSR Ratings, it's even more important to get good shipping prices that don't get you the nasty low stars. On charging, I always attempt to make sure my charge for S&H is at or lower than the actual cost. Yes, I do sometimes lose a little on S&H, but it's also a competitive market. I keep a spreadsheet for running total and averages on the shipping materials, etc., to see if I'm hitting goal.
Perhaps my best response is, see what others do in their auctions/BINs.
I'm doing some similar USPS combined with books using Media/1st/Priority, including some basing of Priority on the Flat Rate Box. However, I VERY CAREFULLY explain my shipping costs and INVOICE combined sales to get the numbers correct. That's due to limitations on containers and book sizes for packing. People love my fast ship and very protective packing methods, so that helps too. At least my DSRs seem to agree with the methods.
You can look at the language I'm using on my auctions for ideas. Just click on my store link.
Did you know USPS is raising the rates again IN MAY?!!!!!