Since they said the part was there and you're finding it was not, then I'd be inclined to open a not as received case (unless the part is of no importance to you or the working of the cage)
My interpretation of their reply about combined postage is that they have agreed to combine it when they send it, in other words, put it in one package, but have no intention of lowering the postage cost or refunding, if that is what you were after.
If you were after a combined postage discount, you might have been better off working it out with the seller beforehand.
You say you guess they mean well. I can't see any evidence of that.
They misled you about a missing part, they said they would send photos & they didn't, they weren't consistent with replying to you-a bit haphazard all round. Plus they didn't give a clear answer to you about combining postage costs. I'm not saying they had to give you a discount but they should have given you a clear answer.
You don't owe them a thing, go for the not as described if you're not happy with the cage fault. The seller may offer a partial refund, which would be fair enough.
