I saw that help thread overnight and was appalled.
To answer the question about whether the billing system is fit for purpose or not...
If you're paying by voucher and buying goodybags directly then yes, I think that works very well.
If you're paying by card and queuing/recurring your package, not so much.
Not gonna lie, I've heard/read so many stories about the company screwing up the financial side that I wouldn't ever trust giffgaff with my card details.
Part of that is due to the tales of unauthorized charges, double charging, payments saying they've failed when they haven't, and so on, part of it is due to the thought of having to contact the agents if one of the above, or something else happened if I were to use giffgaff habitually and give them my card details.
I was gonna run via PayPal to try and reduce the risk of that happening but of course there was a problem and I never actually managed to successfully have PayPal and giffgaff link up so that I could successfully make a payment, (something which PayPal claimed was due to multiple issues on giffgaff's side - whether that's true or not I have zero idea, I'm sure giffgaff would blame PayPal, that tends to be what happens when more than one company is involved, they just blame each other until the customer gives up out of sheer exasperation).
I have the same reservations about Vodafone if I'm honest. The tales of their billing screw ups are plentiful, and I reduce the risk of personally being affected by maintaining a credit balance of several hundred pounds with them.
I'd be more willing to give my information to giffgaff if I could do similar here, (and have the credit balance protection I enjoy on Vodafone), and if support here was even half as good as it is with Vodafone, but sadly neither are true.
My wariness stems from BT continually screwing up bills back in the day which coupled with their appalling support left me in a monthly hell cycle for what felt like eternity.
All that said, like I said right at the beginning, the voucher option works really well, it's just not very convenient.