Hi,
I believe that's entirely up to the hosting provider to select whether or not to support free certificates, especially for those small providers (Although the issues are the same).
Let's Encrypt certificates are indeed also signed by Trusted CAs, but they could easily distinguish that from paid certificates (due to valid length and CA name). I would believe that hosting provider do this for profit. You could try to ask the hosting provider to support free certificate, but it's again up to the provider's mind to accept it or deny it.
The below link is a list of known hosting providers who allow customer to use Let's Encrypt's certificate, most of them would even automated the process for you.
(Ideas are mine)
Thank you