Look we come here with "Lifetime" mindset, but since you can't provide LTD for cost reasons, then at least consider doing this:
The credits we purchase here should last for a lifetime until we have used them, and not expire after a year. This should make your offer here more enticing giving us the relief of "Lifetime" mind, without adding any extra cost to your infrastructure. If you do that me and I am sure many others here will buy.