they are demanding copyright certificate
There's no such thing as a "copyright certificate" (in this context). Any demand by an essay company for payment in relation to a "copyright certificate" is simply an excuse for their blackmail, because "We have to pay for your copyright certificate" sounds better than "We're blackmailing you because we want your money."
Copyright is automatic and belongs to whomever created the work. The creator may choose to register the copyright and apply for a certificate of copyright registration, but its only purpose is to be able to prove copyright ownership in a lawsuit against someone else for infringement, such as for using the work without the copyright owner's permission. If the copyright owner doesn't intend to sue someone for infringement, there's no reason to spend money on the application fee for a certificate of copyright.
Transferring copyright to someone else, (such as to the customer who paid for the work to be created), requires no "certificate" and nothing more formal than anything in writing that says the creator is transferring copyright to the customer. That writing can be as simple as an email or a TOS note saying "customer owns exclusive copyright" or even a statement on a website that says "customer owns copyright to any work" etc. The customer doesn't need any kind of certificate, either, unless the customer intends to sue someone else for using the work without his permission.
All of the guarantees and other promises or representations posted on any website, including about copyright ownership, are always enforceable as terms of your agreement with the service provider, even without any other "writing." Stick to providers whose websites state that the customer owns the copyright to any work and just save a screen shot of that page. That's your proof of copyright transfer from them to you. There's almost no conceivable reason for you to worry about registering your own copyright, but if you choose to do so for some reason, just do it yourself for a small fraction of the ridiculous amount of money demanded by scam essay companies blackmailing you about it. (They're obviously never going to use your money for any "certificate," either, if you're actually gullible enough to pay them.)
So, stop worrying about all these laughably stupid demands about "copyright certificates" because they're nothing more than a way to trick you into believing that those demands are something more than exactly what they are: blackmail. As always, ignore, delete, and block any and all messages from the company and just get on with your lives.