Do email campaigns use up my store's monthly email allowance?

Yes. Campaign emails go out the same way as your order emails and each recipient counts towards your monthly allowance, unless you send through your own Postmark account.

Check Settings → Web Settings → Email Provider before a big send. This month shows how many emails you have used, Remaining and when it Resets (the 1st of the month).

If the allowance runs out mid-campaign, Wcart pauses the campaign rather than marking people as failed, and it carries on once more emails are available. Your order confirmations would stop too, so buy a top-up or upgrade first if you are close to the limit.