first you need to ascertain if the seller is willing to allow you to pick up. many sellers do not, and are not required to.
secondly, as you have absolutely no buyer protection on an item paid with paypal, your seller may not be willing to accept a paypal payment for pick up
thirdly, you cannot remove the postage from the invoice, only the seller can send you a revised invoice if they (1) agree to pick up and (2) agree to paypal for a pickup item.
contact your seller and get their thoughts on the matter.
