Go ahead - if I sell several things to the same person, I adjust the postage on the final invoice (which eBay would normally automatically show (in your case) as £15). The seller changes the invoice before sending it to the buyer. I suggest therefore that you don't pay for the items until the seller has invoiced you, and if she hasn't changed the postage amount, you can then remind her what she's agreed (and I hope she's weighed the 3 items together to ensure that they will go for £8 or less!).
You can't change the invoice (only the seller can do that), but if you click Buy It Now you will have indicated your intention to buy the items (which is then a firm contract) but you don't have to pay for them until the invoice is correct.