If you run out of credits, you can also buy 3 extra for £18 (I'm not sure what the USD price is). That's significantly cheaper than buying 3 audio books at full price.
Credits with frequent 2-for-1 sales, £3 sales and £1 sales, there is just no reason to ever pay retail price on Audible.
One such example might be accessibility settings. Such settings are likely only used by a small subset of users, but it might be critical for such users.
Many modern video games start by displaying such settings. The defaults probably suit most users just fine, but being able to easily scale UI text is probably very useful for people with poor vision.
They do '“envisage” offering its services to users in the EU'. That page has a massive ad (700x500 px) suggesting that I buy a subscription for "DKK10 FOR 3 MONTHS".
If offering their service priced in the local currency of an EU member state doesn't constitute offering services to users in the EU, then I'm not sure what could possibly qualify.
Credits with frequent 2-for-1 sales, £3 sales and £1 sales, there is just no reason to ever pay retail price on Audible.