Can't you just accept the immediate cash payment, but make sure that it's recorded correctly in your accounts.
I don't know where you live, but the tax law I'm familiar with doesn't care if the customer received a bill in the post or even if they recieved a bill at all. The critical thing is that all payments received within the tax period are declared.
You could still produce a copy of the bill for your records and even a receipt for the payment if it makes you feel better.
Peter File
EDIT: Source "The IT Crowd"
It says 315,000 dead and injured.
You're coming across as if you're saying that all anyone with ADHD has to do is try harder and their symptoms will be gone.
I would like to imagine that you're actually trying to say that there are strategies that can help with improving the symptoms of ADHD and that finding and doing ones best to employ these strategies is more productive than just complaining about these symptoms.
Unfortunately though, ADHD is a chronic neurodevelopmental condition which is characterised by its associated symptoms. These symptoms will never disappear as a result of 'trying harder'. If that were the case then the diagnosis of ADHD wouldn't exist.
Simply telling someone with ADHD to put more effort in is insulting and can be quite hurtful.
It is also unhelpful to single out those who might be awaiting diagnosis as 'self-diagnosed twats'. Since you were quite young when you were diagnosed it's understandable that you likely do not know what it's like to go through the diagnostic process as an adult. The constant self doubt and imposter syndrome that nags away at you while at the same time you know that the symptom list reads like a brutally honest biography. Having others look down on you just because you haven't yet received the diagnosis yet is very hurtful, unfair, and unnecessary.
I'll add another vote for Podcast Addict.
The First Testament in the Christian Bible (the entire first half) is the Torah, or Jewish Bible.
That's such a great reaction to starting a fire in a closet full of priceless and flammable stuff! "Oops! I think I'll just close the door on that problem and hope no one notices."
I'm tempted to call it such a child's reaction to a problem they don't know how to solve. But I know I'm guilty of doing the same thing as an adult, just not with a potentially fatal raging closet fire fueled by a plastic coated wedding dress.
The more I think about it, the more in awe I become of what you managed to achieve.