It costs 7 NEAR to deploy a store, this goes to the NEAR blockchain and not Mintbase. This is a storage costs as you are paying to reserver space on the chain. You can see 602939 Bytes is the storage used for the soul4space store:
And if you click the Balance Profile you'll see there is 0.5 N left. Each time you mint, you take up more memory and this number will shrink. You just need to send more NEAR to the address soul4space.mintbase1.near and you'll be good to go.