with the widget nothing is on your site. it is just a crude window into shapeways sight.
AJAX calls(which is what shapeways uses way to much) can not be executed across different domains. As a fix a simple proxy php script needs to be writen.
Unlike the widget which will work with absolutely any provider the buy now option would require a hosting provider that allows PHP(most pay some free). Shapeways could auto generate the code by defining commands like this:
Code:
<? DrawThumb( MODEL NUMBER HERE );?>
<? MakeLink( MODEL NUMBER HERE );?>
in this way you could make a page listing products 1,2, and 3 in the same thumb nail shape as the widget by writing the following code
Code:
<? include_once "shapeways_buynowapp.php"; ?>
<? DrawThumb(1);?>
<? DrawThumb(2);?>
<? DrawThumb(3);?>
The first line opens shapeways buy now app. Place on the first line of your page(make sure your page as extension .php instead of .html and the remaining lines open each thumbnail.
Of course they would need to spend a few hours and write the API first but would be easy to do. And you would have to upload the file they give you to your site.