You dont have to sign up for a monthly membership.
if you just wanted one time help, thats fine.
It sounds like you were on pay-as-you-go pricing.. In which case, mail chimp doesnt offer refunds for campaigns that were cancelled.
However, Im wondering why you arent making use of their free service. You can send up to 12000 emaills a month for free, as long as you dont have over 2000 subscribers / recipients to send to.
It seems like all you need to do is add more credits and use them. But again, if you arent sending over 12000 emails a month, you should look into their forever free plan as that can save you a ton of money over the long run.
If you have further questions, let me know.
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Senior Technician and Network Switching Systems Operator / Maintainer.