From their docs[0] this doesn't seem to apply if using ACME, but they don't exactly make that clear...
reply> By using ZeroSSL's ACME feature, you will be able to generate an unlimited amount of 90-day SSL certificates at no charge, also supporting multi-domain certificates and wildcards. Each certificate you create will be stored in your ZeroSSL account.
Yeah, they don't make it that clear, but you get basically the same functionality as with LetsEncrypt for free, including wildcard certs. You basically only need to pay for manually issued certs, or some of their other additional features.
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