Are there any VPN services which sell X hours of service instead of pay by the month?

When travelling, I would like to use a VPN when I’m on unknown networks. There are many VPN services out there which will encrypt all traffic, but all the ones I can find seem to only have monthly or yearly pricing. Instead, what I would like would be to pre-purchase a set number of hours (or gigs) of VPN service which I can then use whenever I need. It may be months between when I travel and I wouldn’t need VPN during that time. Are there VPN services out there which sell a bulk service package instead of a subscription?