Chose Donate Via Paypal option on libreoffice.org. Got a Paypal input screen with a preset amount ($9.00) that couldn’t be increased or decreased. Went to screen multiple times, each time amount displayed varied. Finally made 2 donations, one for $9.00 and one for $14.00, to make desired $25.00 contribution. Incredibly annoying. You need to fix this mess.
I think you should go to http://www.libreoffice.org/get-help/feedback/ and report this problem in a bug report. It will take a bit of stumbling around to figure out how to do this the first time, but I promise you will be using this again once you use it the first time. (Also get there from Help | Send Feedback) in your LO menu bar.
Thanks for reporting this and thanks a lot for your donation!
I just gave it a try and could change the pre-defined amount on the LibreOffice donate page without problems.
Once it’s handed over to PayPal, you might not be able to change it if PayPal doesn’t permit, but you can choose the amount on our donate page.
Can you try if it works?