According to this site, it says prepaid tablet/mobile broadband devices which is more than 12 months in service, says no fee to unlock.
When I put in my info to unlock my E5251 which I've had it since 2015, and recharged it more than $200, it tells me the fee to unlock is $25.
There seems to be some vagueness around if the recharge needs to be a continuous period. But I don't think that's the issue.
Presumably just a glitch. Have you tried entering the details here?
If it still asks for money then perhaps pop into a store or ask LIVECHAT.