If you have been using QuickBooks Online, it is likely that you already have several thousand invoices. In this case, you'll need to ensure that the invoice numbers in Housecall Pro do no coincide with the invoice numbers already used in your Quickbooks Online account. Quickbooks Online does not allow for more than one invoice to have the same invoice number.
To do this, you'll need to set the invoice numbers in Housecall higher than any invoice number that already exists in your Quickbooks Online account. You can do this by creating your next job, going to the job details page and click directly on the invoice number listed to the right of the 'schedule' button. You can then enter whichever number you'd like the invoices to start from. This number should be greater than the highest invoice number that you have in QuickBooks Online.
Once the number is set, any invoices created from this point on will continue in sequence from the highest number that is set in Housecall. This eliminates any possible errors with invoice numbers in QuickBooks Online.