Advice on shipping lots of cards

I want to do large trades of bulk for bulk to complete sets. The problem is that by the time I pay to ship 50-100 cards, even if those cards were super low value, I would have been better off putting that shipping cost money toward buying the bulk I needed (and keeping my bulk). Does this mean it's only really ever worth trading when you are the one shipping fewer cards (e.g. in a plain white envelope), and RECEIVING many cards? Is there a secret to efficiently trading large quantities of cards (by large I mean enough that it needs a box) for large quantities of cards?

Do folks have a rule of thumb for when the trade value merits shipping costs? e.g. if it's going to cost $10 to ship a box, how high does the trade value need to be for that to be worth it (with the alternative being just buying the singles on cardmarket? $50? $100?

Do folks have rule of thumb for how to price shipping? e.g. 100 cards costs X, 500 cards costs Y, etc.

Thanks in advance for any help or ideas you're able to offer!!!