You probably won't be interested since it's even more pressure to use them before they expire, but there's also annual plans which are even cheaper if you can be happy using 12 (or 24) tokens within 12 months (you get them at the start and they expire at the end of the year):
Audible Premium Plus Annual - 12 Credits $149.50/year (way cheaper in UK: £69.99/year)
Audible Premium Plus Annual - 24 Credits $229.50/year (£109.99/year)
US: https://www.audible.com/ep/memberbenefits UK: https://help.audible.co.uk/s/article/what-are-the-different-...
Although, as soon as I'm logged in with my account (UK) which had subscribed in the past but isn't currently, it doesn't seem to be giving me any options except to start a 1 month free trial for 1 token/month, not sure if other options aren't available or just extremely well hidden...
edit: no it is available for my account, though I'm going to remain a non-subscriber and keep using my local digital library :)