I've noticed that there is a standard 30-day money back guarantee on certain things online. I am not entirely sure why this is.
A sales tool in most cases - it gives customers the confidence to purchase your services.
The service will be to work on existing websites and install certain premium plugins. Unfortunately all these cost money, so giving a refund would cost me quite a bit of money. Furthermore some of the services I'd be using don't give back refunds, so this leaves me in a hard spot.
TBH if you intend doing this as a business, you might want to consider buying multisite/developer licenses for these premium plugins; most plugin developers offer this option, so refunds become a moot point because each license doesn't cost you anything 'per client'. Or talk to the plug in developers about license transfers - if you refund a client, see if you can transfer the license you bought for them to the next client who wants it.
Regarding refunds in general, IMO as it's a service, it's no different to performing any other type of web development work so you could certainly say in your contract that refunds cannot be given, but that doesn't stop the client asking for one, nor demanding it through their card issuer or the courts (assuming Cypriot law is the same as UK...).
I think you'll find that refund requests would be pretty rare, especially if you handle customer expectations well early on in the process. How likely do you feel customers would be looking for refunds, and for what reasons?