Hi, I am kind of a new seller but i'm sorry i might not be of much help. I offer combined postage too but this how i do it,

1. i write in my listing that if they buy multiple items to wait for an invoice from me before paying as i offer combined postage.

2. I then weigh all the items on my kitchen scales.

3. I then work out by how much they weigh what aus post package they will fit into (even if it's just a letter size because i sell small items).

4. I then send an invoice with how much the postage would be for all items combined in one package.

Edit: i mean i weigh the items they buy, not all in my listing.