At this point, you have two choices. (You cannot force the seller to alter his invoice to honour the original price.) His behaviour is prohibited by ebay but there is not much to actually be done. You will either have to ask to Cancel your order if the extra $5 is too much for you, or pay it and then challenge him when it arrives about the extra charge.
I don't know which of the two undesirable options is better. I suppose it depends how badly you want this item. Enough to pay an extra $5?
Regardless, make a note to never buy from that seller again. There are dozens of great sellers on ebay for every bad one like this.
p.s. This isn't a Canadian seller charging tax on the item, is it?