If you want to pay with Paypal or TransferWise, include in your ticket which upgrade you want to buy and for how many months. You get a discount if you buy 3 or more months.
You will get an address or a link to send it to, and you will receive coupons for your upgrade. Each coupon equals one month upgrade. You can chose to use them all at once for a consistent upgrade, or spread them out over time.