Please specify your relationships. It sounds like your problem may lie there.
In particular: " the Invoice only pulls over the first line item from the Order Portal"
What does that describe? A portal to a LineItems table placed on your invoices layout or something else?
The table "Line Items' is related to the table "Subscribers'. The invoice is a Layout form. The information that is placed on the Invoice is derived from the same fields as contained in the Subscriber table. Does that make sense?
I also just realized a design flaw.(I think) If a Subscriber comes in after purchasing their season subscription and wants to purchase some single tickets, the layout in the Subscriber table will calculate all entries in the portal and not just the new ones.
Sounds like you need an invoices table.
Invoices::InvoiceID = Lineitems::lineitemID
Invoices::ContactID = Contacts::ContactID
For each new sale, you'd create a new invoice record and list the items sold in the line items table. To link to a customer record, you'd select a contactID from the contacts table.