I am not sure of the first question kinda'. As sasta10 pointed out Paypal and other European companies like Payson.SE in Sweden are a 3rd party handling money transfers electronically. The sellers do determine payment options and criteria.
An example is Amazon will not invoice your credit card or payment method until the item is shipped. Many sellers have this as a policy. A difference is like eBay sellers who have their own criteria again. Many individual sellers and company sellers will receive the payment before shipping.
Regarding the first question I have ran into both cases with businesses or selllers. Some have debited my card or account at the time of an Advanced Order as a pre-payment and others again only when shipped. Paypal really has not any kind of an option similar to a lay-a-way style themselves either other than of course their credit card.