Hi there,
Thanks for taking the time to submit this suggestion.

A single invoice in WHMCS can contain potentially hundreds of different services, and the field length in PayPal is limited to 127 characters: https://developer.paypal.com/api/orders/v2/#definition-purchase_unit
Presently WHMCS adds the Service ID the custom_id field, and the invoice ID in the description, which we feel is the best way to handle such as one-to-many relationship.

Happy to hear alternative suggestions which meet the technical specifications.