Enter your idea

The underlying issue is that Xero treats expenses and invoices as two separate records:

The expense record in Xero Expenses (Xero Me), which holds the receipt image/PDF.
The sales invoice to the client, which holds the billable line item.

When you on-charge the expense, Xero links the accounting transaction but does not automatically package the attached receipt into the outgoing customer invoice. The billable expense functionality is primarily designed for cost recovery and audit trail inside Xero, not for client evidence packs.
For a consulting business like Surge, this creates a practical problem:

You capture a Virgin receipt in Xero Me.
You mark it billable.
Xero adds it as a recoverable cost on the client invoice.
The client wants supporting evidence.
You must then separately retrieve the receipt and attach it.

That's a manual workflow and duplicates effort - hot tip - just allow the supporting evidence from the Xero Me (expense system) to be attached in the billable expense process.