Are you allowed to issue certificates for domain/subdomains if there is any money involved?

I got in a bit of a conversation with StartSSL about a domain that has a donation button and a small shop on it.
I process the Bitcoin transaction myself and offload any other transaction to PayPal.
Is that allowed under Let’s Encrypt?

Sure, you’re not messing around with an unbounded number of subdomains, and I assume you aren’t expecting people to access your server via an IP address, so you should be 100% fine with the normal offerings.

Yes, Let’s Encrypt has no noncommercial use limitations or restrictions. It is permissible to use Let’s Encrypt certificates with commercial or for-profit sites, including sites that process or receive payments or financial transactions.

Some payment processors might have additional policy restrictions about server security, but there are no such restrictions from Let’s Encrypt’s side.

I am not doing what I do for profit, I just allow people to give me money if they want to and I rarely get any money.
I usually pay for everything myself.