If you get a error when trying to upgrade or install that is "similar" to the following"The formula references fields across 16 unique relationships while only 15 are allowed. Please contact support at salesforce.com for more assistance.Subscriber_Group__c.Subscription_End_Date__c: The formula references fields across 16 unique relationships while only 15 are allowed. Please contact support at salesforce.com for more assistance".
Payments2Us does NOT exceed the maximum number of allowed cross reference formula fields as part of the standard package.
If you add your own formula fields, then the maximum can be exceeded. As per the error message, you will need to log a request asking Salesforce to increase the number of cross reference formula fields.
You should do this for production. You can ask Salesforce to apply to the sandbox, but may need to do a new refresh to pick up the limit.
Please see this Salesforce help article for more information.