The PayPal payments page can be accessed via the below url, it is in the public domain so it doesn't require a login to access.
For this to work for your library will need:
For Example Maroondah Toy Library would have a url https://maroondah.mibase.com.au/public/paypal/index.php?m=NEW&&a=10&&id=758
There are default values in the URL that define the amount, type of payment, and member id
You can include default values for the payment in this url in any of the templates that has a [renewal_fee] and/or [borid] bookmark.
For example if you were to include a link to the payment page in the due_to_expire automatic email, it would be
These defaults can be changed or added to by the member, via the form under the paypal button. To add detail, the use r can select the payment category or edit the reference detail.