Unless you have not blocked bidders from editing your invoice, the shipping on the invoice is what you put in the listing. It doesn't change. So if you put $3.99 shipping in the listing, it will show up on the invoice and the buyer cannot change it. Whatever shipping you listed, that is all you can charge.
As mentioned in your other thread, you may have meant to have US shipping $11.83 and Canadian shipping $3.99 but if that is not what was in the listing (if I'm right about which listing you are referring to) then the buyer doesn't have to pay anything more than $3.99 for shipping.
Since the buyer has paid the shipping according to what was in the listing, you will need to ship the item.
Non paying buyers deserve unpaid item strikes.
