giffgaff SIMs need to be 'activated' before they can be used. Activation is done online and requires a top up via either a payment card or top-up voucher.
Unfortunately, giffgaff do not currently accept non-UK cards, and obtaining a top-up voucher from outside the UK can be difficult.
If you add a voucher to your order, there is no need to send any money until AFTER you have received your SIM. If you are not sure about what to do, simply leave the box unchecked and let me know once you have made your mind up.