I have a customer who uses Bill.com, can I send them an invoice so that they can add it into their Bill.com account system and reconcile it in their Bill.com account?

Yes, you can send an invoice to your customer manually via email, Luke. However, I suggest they consult Bill.com support for instructions on how to enter and reconcile the transaction within their platform. 

You can only integrate your own Bill.com account with your QuickBooks Online (QBO) account to facilitate faster payments and receivables.

Your customer's Bill.com account is separate from your QBO account, so direct integration between the two isn't possible.

In this case, I recommend advising your customer to contact Bill.com support for guidance on how they can configure their account to accept invoices from external QBO users.

If you have additional questions or concerns, feel free to comment below. We're here to assist.