How do I handle billable travel expenses with HST if I am based in Quebec serving a Quebec business?

Good day, michael-partica,

You can add provincial tax rate for Ontario to track sales tax. Here's how:

1. Click Taxes on the left pane.
2. Select Sales Tax.
3. Click Manage sales tax.
4. Click Add tax.
5. Click Add on Provincial.
6. Select Ontario.
7. Add.
8. Done.

Once added, HST 13% tax rate will show on your tax drop-down the next time you create a billable expense. 

Check this article for future reference: How to enter billable expenses.

Let me know if you have additional questions.