How do I apply a billable expense to a sales receipt?

It's super cool that you answered this, but your answer doesn't actually address the underlying issue...

As an accountant and ProAdvisor, this is something I deal with all the time and, frankly, it's a limitation of the software that needs to be fixed. 

Let me first acknowledge your answer: yes, you might invoice your client for reimbursable taxi expenses incurred while meeting with them. Alternatively you may have an agreement with your client to automatically bill them their statement balance at the end of each month and ... this is where the issue of not being able to add billable expenses to a sales receipt becomes a huge issue.

Invoicing is not the only way, nor is it necessarily the preferable way, to bill a recurring client you've established vendor terms with. The sales receipt is a totally acceptable way and is, indeed, preferable in many scenarios.

So, to fully address the OP: yes, billable expenses should be available to be added to a sales receipt and, no, QuickBooks Online does not currently offer this functionality. This is an unfortunate limitation of the software that should be corrected.