The order data should take the form of additional records rather than data appended to the same field.
Your import should add new records to lineItems and orders that are linked to the same customer record.
Thank you @PhilModJunk — not crystal clear, but seeing some wisdom here. My database is currently just simple text fields, it sounds like I need to overhaul to allow for lineitems?
Will research into this. Still welcoming explanation from any helping hands. Thanks!
What you want can be done, but it works from a very limited structure that won't let you do as much with your data as you will be able to do if you use a related table of records where Shirt1, Shirt12, Shirt5, Shirt7 are different records in LineItems lined to records in orders linked to a record in Customer.
You may want to examine how the Invoices starter solution is set up. It's line item table is named "invoice data", but the relationships that I am describing are used in that starter solution.