You received a €100 invoice because you hit WPML’s automatic translation safety limit.
When using pay-as-you-go, WPML charges you immediately when you spend €100 in automatic translation credits. This is different from your regular billing, which happens at your normal billing period. After this €100 charge, any additional automatic translation spending will return to your regular monthly billing cycle.
WPML has a second safety limit at €4000. If you reach €4000 in automatic translation credits, WPML will charge you immediately again. Like the €100 threshold, this is a one-time immediate charge – after that, future spending returns to your regular billing period.