The monetary charge in question is an arbitrary amount charged by the store to verify whether you can make valid payments after you have registered in the store before the first actual payment.

No additional fee will be charged after the $1 (dollar) payment has been confirmed.

[Link to Arbitrary Payment Guide by Google Play Store]
※ Please refer to the item in the above link regarding verification.

[Link to Arbitrary Payment Guide by Apple App Store]
※ Please refer to the item in the above link if the small charge amount is displayed as pending.