If you allow some customers to pay for multiple months of their route stop/maintenance services in advance, you can create a payment (or account credit) so it gets auto-applied to future invoices automatically.
Create a payment and set it to "auto apply balance"
Create the payment in Pool Brain by going to a Customer page --> click "Payments" tab at bottom of page --> click "Add New +"
Make sure the checkbox to "apply balance to next created invoice" is checked when creating the payment as shown here:
After the payment has been created with this setting, it will auto-apply to each route stop invoice that gets auto-created every month until the payment balance is fully exhausted. You can also do the same thing with an account credit instead of payment if needed but in most cases, payment would be correct to use.
Note that this will only happen for auto-created route stop invoices and not manually created invoices or invoices auto-created from one time jobs. However, you can still manually apply the payment to any invoice you like at any time.