Do I really pay once and store forever?

YES!

Your payment goes into a storage endowment. It's like the principle upon which the interest goes towards paying the future storage of your data. Indefinitely.

You can read more about the Arweave storage endowment here:

To break it down into what this means in a very practical way: if you have 1 GB of storage on Akord and you upload 1 GB's worth of photos, you never have to pay again to keep those photos on Akord and on the Arweave blockchain. Even if you don't login to Akord for many many years, that data will be preserved and waiting for you.