I am assuming the typical set of tables and relationships for Point of Sale:
Where you are populating records in a portal to LineItems on an Invoices layout.
Write this script:
Set variable [$InvoiceID ; Value: Invoices::InvoiceID ]
Go to Layout [Line Items]
Set Field [LineItems::InvoiceID ; $InvoiceID]
Set Field [LineItems::ProductID ; Get ( ScriptParameter ) ]
Go to Layout [Original Layout]
In button setup, specfy that it perform this script and that it pass the ProductID to this script in the optional script parameter box.
For each additional service that you want to set up with a button, add a button to perform the same script, but pass a different ProductId as the script parameter.