I'm not sure if I understood correctly, however if someone has your WHM access they shouldn't be able to buy something from the cPanel store as they would need your cPanel store logins, or they would require your login details for the billing platform of your host to do any harm.
I agree with @arlindmurati1 - while having the WHM access stolen is never good, and would necessitate a migration to a new machine that is secure, they would not be able to make purchases without also having the cPanel store account.
I agree with @cPRex a migration is an immediate action you should take, even better from an offsite backup, after restoring please make sure to change WordPress login details, database names, database users passwords, cPanel passwords, disable mysql remote connection etc..