PHP include to add a website in a Wp page (without destroy layout!)

Hi!
I’m trying to add my Spreadshirt Shop in a wordpress page! I don’t want to use iFrame because this will cause a lot of problems with the Google crawler, so I thought to use PHP include (if you know a better solution, please tell me!). So I created a new template including these lines:

<div style="overflow:auto">
<?php print file_get_contents('http://ohkay.spreadshirt.it/index.php'); ?>
</div>

The code works but it completely destroy the layout of the spreadshirt page and the layout of the wordpress page!

Could you tell me please how can I include the shop in WordPress without have this problem? Thank you very much and sorry for my bad english (I’m italian!)