I removed domain registrars so I could only sell hosting and not register new domains.
The order form works, however the area to enter the user's domain only appears if you click the domain checkbox (I will use my existing domain and update the nameservers).
This selection must be deselected by default so that when the customer clicks on it, the area to add the domain appears.
As it is, it is already checked but without the area to add the domain, even though the option is checked (I will use my existing domain and update the nameservers) and if I click next it says that the domain is invalid, so in the area to To add the domain I have to click again on the selection that is already marked, to appear the domain area.
This may confuse some customers, it took me a while to realize this.