You can use vouchers exactly as you are trying to do. There are some people that ED and link are helping out that lost money due to past projects led by another person. To make sure there are funds to actually make the Pyra those special cases have to wait to use the vouchers related to those other projects for their Pyra.
[doublepost=1469519084,1469518910][/doublepost]And to be more clear in what you are trying to do, PayPal does not permit preorders, so the only way to preorder with PayPal is to use shop vouchers purchased with PayPal to pay for the preorder. Again, everything should be fine with the way you are doing it.
thanks for the explanation!