Hello 'limenglim',
Did you give your seller a reasonable length to respond to your query?
Did you ask about combined shipping and confirm an agreed price before you bid?
If you agreed a price with the seller before you bought the items, you need only send a
sweetly worded note to the seller reminding him of the shipping fee you both decided on
and asking him to please send an invoice with the adjusted amount so you can pay it
right away.
That should do the trick. After all, sellers like to be paid.
And when you get the adjusted invoice, be sure to pay it right away and thank the seller.