Campayn does have 'Pay As You Go' plans to suit your email sending needs. Pay as you go plans never expire.
PAYG is good for occasional burst of Emails, it’s not good for regular scheduling, for that purpose you should consider a monthly plan.
Credits work like this:
1 credit = 1 Email sent to 1 contact
In more detail, if you send 1 Email to 500 contacts, that’s 500 credits worth.
You can find our Pay as you go pricing on the pricing page.