@pcal, several accounting factors could influence how you do what you want:
How often do you invoice and expect payment? If you sell 2 years’ service at once, you could issue a sales invoice and recognize the revenue then. The disadvantage, as you suggest you realize, is that there is nothing to show you owe delivery of the service. So you could, instead, have them sign a contract and make a payment that you allocate to
Customer credits, without issuing a sales invoice at that time. Then, set up recurring sales invoices to be issued monthly or quarterly. Manager will automatically “pay” the invoice from the customer’s credit. The balance in
Customer credits would be an indication of total future wholesale requirements, but would not show their time phasing.
If the customer does not pay in advance, but periodically, use the same approach of recurring sales invoices, but without the intervening step of customer credits. This approach gives no indication on your books that you must provide the service. But that problem is no different than a carpenter has, who must decide how much lumber to buy, or a chef who must decide how much fish/meat/vegetables to buy each day. You will need some estimating/forecasting mechanism outside of Manager. A simple spreadsheet incorporating a chart would do that job.
Your idea of using inventory doesn’t solve this problem, because inventory is not predictive. It records how much of something remains. From an accounting perspective, the fact that you’ve agreed to provide something in the future is not really a transaction, but a contract. Exactly how you handle it will depend somewhat on whether you are doing accrual or cash accounting. Under cash accounting, you can’t account for your future obligation at all. Under accrual, you might create liability accounts, just as you would for a loan. You should probably consult an accountant on that. One issue will be what your customer’s recourse is if you should fail to deliver the service? (A bank loan would probably be collateralized. Internet service would not be, so there may be no actual financial obligation beyond return of any advance payment.)