tl;dr version: You sign up for a monthly recurring payment and in return HP monitor your ink levels and number of pages printed, and mail you out new ink as you need it. If you print more than the included pages per month they will charge you an overage. If you stop paying the monthly fee they revoke access to the ink you already have in your procession (ink provided by the subscription service - they don't revoke any ink you have purchased from your local store for example, just to clear.).
My advice, If you only need to print once in a blue moon, get a refrub'ed "office grade" laser printer off eBay (or similar). I picked one up about 10 years ago with 64,000~ prints on it for about £50, and apart from having having to change out some foam pads that had disintegrated over the years, its still printing just fine. Only had to refill the toner once (I don't print a lot) and because the drum and fuser were fine I just ordered a bottle of toner refill.
EDIT: Reread your post. As you are only printing a single page every couple of years, I would adjust my advice. You might be able to ditch the printer completely, When you do need to print use a local library or similar to do the one off print. My local library charges 15p per side for black and 50p per side for colour prints.
Sure I wouldn't use them to print large amounts or even small amounts regularly, but for very very low usage paying a quid once in a blue moon to use someone elses printer and then you can leave the maintenance of the printer to someone else. But granted, you would then have to travel to a place just to print, and if you need to get the document printed ASAP, it might not be the best thing for you. But its another option to consider.