You can buy a £5 goodybag with a card though, just there aren't £5 vouchers, & it's not possible to buy straight credit for £5. I haven't used the Paypal option, but I would guess you could also buy a goodybag for £5 with Paypal?
Yes you can buy £5 goodybag via PayPal.
I use PayPal to buy gigabags for my iPad and goodybags for various members of my family. Sometimes I have bought the £5 bundle, sometimes a larger one. Currently I am using the £7.50 gigabag on my iPad. It is only Vouchers which are restricted to £10+ and to available denominations. Both O2 and giffgaff have stated that £5 Vouchers are too expensive to process as a general thing, which is why the many suggestions for a £5 Voucher have fallen on deaf ears.
Purchasing outright by PayPal or debit/credit card is possible, without price restrictions. Minimum credit topup is £10, but you can then use part of it to buy a smaller goodybag, and it is useful to carry even a small balance for anything not included in allowances.