Unlike function LIST, function LIST_LINE does NOT insert line breaks. Please read the User Guide carefully, it is clearly described there.
If you want to let the user pick instances to appear in the text file, you can do it with a process like this:
PICK ONE OR MORE FROM PatientBilling WHERE ...
CREATE BillingBatch
INSERT PatientBilling IN BillingBatch.Items
EXPORT DOCUMENT BillingFile TO FILE 'C:/billing_file.txt'
BillingFile here is a document template of type Text that has a single tag in it like this one:
<<LIST_LINE(BillingBatch.Items, ...)>>
This is standard Aware IM functionality described in the User Guide.