How do you get MS Cashback on multiple items from same seller when combining the shipping?

I am looking to buy multiple items from a seller. Will I get MS Cashback by waiting for an ivoice from the seller to give me the figure of combined shipping? Should I just pay for each item separately and forget about the combined shipping?
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BST: Tennessebanker, Downtown1974, LarkinCollector, nendee
BST: Tennessebanker, Downtown1974, LarkinCollector, nendee
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WTB: 2001 Leaf Rookies & Stars Longevity: Ryan Jensen #/25
I know I must go back to Live.com each time I make a purchase. Let's say I do this three times. Now I would have 3 items with one seller. How do I pay? Do I pay for each item separately or wait for an invoice with all three items on there?
BST: Tennessebanker, Downtown1974, LarkinCollector, nendee
WTB: 2001 Leaf Rookies & Stars Longevity: Ryan Jensen #/25
BST: Tennessebanker, Downtown1974, LarkinCollector, nendee