There are quite a few discussions on here which relate in some way(s) to DIYs, but they might well not cover the points you need. So here are some pointers:
The DIY Orgs (including me!) certainly take PayPal, see
https://www.aukweb.net/forms/diycards.phpIf you want to use the route of "Tasty Cheddar" as a DIY:
It would be proper/polite to mention to the Organiser of the Calendar Event that you hope to do so; perhaps you entered the event but were not able to ride it (today).
You will need to go through DIY entry procedure, I'm afraid that (as you suspect) you can't just say "doing the Tasty Cheddar 100". There's a lot of information on the AUK site about ways of entering DIYs, all likely to be helpful but, as you've noticed, it can seem daunting at first. It sounds as if you will prefer the "Mandatory Route" option - it does have plenty of advantages - so, on the entry form, use the "File Upload" to put your file in (.gpx of Tasty Cheddar, or your own version of it starting from home perhaps), and list some controls somewhere on the route - "about every 50 to 80 km" it says, so you need only one intermediate control on a 100, as well as start and finish.
Email me about stuff you'd like clarifying, and/or (and probably better) put in an entry, even if still feeling daunted, and I'll let you know if it needs adjusting.