It's the repeat nature - asking for the same documentation over and over again - that's so annoying here.
"4/ Our bank statements are none of your business. 5/ The details of our venue agreement and insurance are also none of your business."
So they're refusing to prove it's a conference but requesting that Paypal are "reasonable"? Having your account verified once and then go dormant in payment receipt for the majority of the year then start again would seem suspicious to me... especially when their justification isn't something they're willing to prove.
But they have proved that they are a conference for the same dates for the previous three years.
And the implication is that Paypal are asking for more documentation than previous years ("the documentation you are asking for this year...").
Even assuming paypal aren't crooks - is doing this OK with the Scottish Data Protection Act, Charities commission or Company registration?