Pay for what you use.Nothing more.
Prepaid credits, no monthly subscription. Credits do not expire, and a generation that fails is refunded automatically.
Questions
The short answers
No. Credits stay in your wallet until you spend them.
Topic suggestions, the content calendar, and adding websites. You can manage as many client websites as you need without spending credits.
The credits are returned to your wallet automatically. A post that comes back unusably thin is treated as a failure too, so you are not charged for it.
No. Buy a single credit to try it, or buy in volume. There is no monthly fee either way.
Checkout presents your local currency where it is supported, and the exchange is handled for you. The account itself settles in euro.
Tax is calculated at checkout from your billing country. If you are a business you can enter your VAT or GST number, and any reverse charge or zero rating that applies is handled automatically.