Potential seller's strange policy

Is it normal when a potential seller requires that I make a PayPal payment for the item, then, when he receives the payment, he will pack the item, calculate the shipping cost and issue another invoice for shipment?

In such a case, I would have to pay a PayPal fee twice, and I am not sure what form of shipment he may choose, though I informed him of my preferred form of shipment.

I feel uncertain - usually, the shipping cost and the form of shipment are agreed upon in advance and paid once.

Any comments?