It could be that your statement about loving to offer true combined post is putting them off. That statement could be interpreted as meaning that when they go through checkout the right postage will automatically show without them needing to ask for an invoice.

Ebay has obviously opted you into immediate payment required because when I clicked on BIN on one of your items it took me straight to checkout rather than confirming my purchase first.

I tried putting three items in the cart and up the top right it says "request total" but it's not clickable. Next to it is a small i in a circle and when I hover over it, it says "You can't request a total because the seller has already offered a discount for these items." Unfortunately the request a total button doesn't always work and the buyers get conflicting messages that just leave them confused.

Perhaps your promotional discounts are interfering with the cart but it doesn't work for a lot of sellers who don't have the discounts. I use letterpost most of the time so I've stated in my description how much postage will cost and said that I'll refund any overpaid postage. I've decided to just wear the extra fees on postage but it's much more complicated when you use parcels because you're refunding much higher postage amounts that you've already paid fees on. I think the only way is for you to offer to set up special listings if they want several items, but with what you're selling a lot of buyers won't bother to ask.