Good day,
I have over 20 years of website design/development experience and about 10 years of experience on CMS's like WordPress & others. I am happy to work for you on this project.
I won't do into much details about features that is because you listed all needed. One thing is that the websites you mentioned as examples does not perfectly fit into your request, but of course this can be done.
To summarize, you need a user to select a shirt style/design then size/measurements then fabric color/pattern then fabric type then buttons; or any other order then they should be able to save, share or order the finished design.
Of course when they save or order/save, you will get all the details the user inputted including an image of the final design. You can provide previously created designs as gallery/library of designs.
I am ready to do the work, this concept is not requested daily, so getting a pre-done example may be difficult, but after basic agreement with you I will provide you with a demo that may show some of the requested features to show you that this is possible and then working on making it happen.
As a free addition, I will make your website SEO ready.
As a free addition, I will submit it to major search engines (goodle, yahoo, bing).
As a free addition, I will link your website to social networking websites.
Best regards,
SolutionsSP