Even if your customer is not onsite to pay the bill, you can still take care of any outstanding invoice by sending an email with a link to pay...and X-1 does the rest!
Here are the two ways in X-1 to request payment by email and have your customer pay their bill... from anywhere.
From the invoice
Select the 'Ready for Payment' option under the caret (^) symbol next to Payment button
2. Once invoice is in "ready" status, select "Request Payment by Email"
3. Use the email template that prompts with the link for your customer to pay invoice.
From the action icon (outside of the invoice)
Another way to request payment by email is from the Invoice List screen, using the squares action dropdown: