Do I have to pay for the valuation?

No, the valuation is completely free.

What can be changed in Blogger themes designer?

It all depends on the template and its elements, but mainly it is: font and color of the main font, titles and menus, bolding effect, tilt, strikethrough and underline text and links (visited, before and after hovering), background color of the blog and its elements (like the side-column background), a photo in the background.

What pages can have a different look?

Home/start page, page with posts, page with post after clicking read more, page viewing posts with labels (each page with a label can look different), bookmarks/subpages of the blog.

Can I place an order for a blog that does not exist yet?

No, I don't make templates for completely non-existent blogs, but there is no problem if the blog started a second ago or is currently closed and waiting for the official opening. The blog must contain most of the gadgets you want to have in your order, subpages and at least one post. For the gadgets you want, but you can not add (eg, slaider), texts/pictures must be prepared.

Where to write about the order?

On my email or facebook.