Each service is different. Go to services and take a look at what is ancluded in each sercivce. 
Extra osts are yur domain cost and your Showit monthly subscrption. you can check both here:
Showit
Domain

what is included in the price and what extra costs are involved?

Yes, all templates are customizable on Showit. After purchasing, you will recive a tutorial video that walks you throgh how to change your colors, texts, photos, etc.

are your templates custumaizable?

Yes and No! Normally you are the owner of your domain everything will be transferred on your off-boarding day, so you have access to the backend of your wensite and in full control of your domain.
BUT if you are using payment plans, the domain and showit backend will remain in our property and as soon as your invoce is paid off everything will be transferred to you. 

Do I own the domain and do i have access to the backend of my website?