How to integrate opencart on a 'Donations' page

I am working on a new website for an animal charity but now that I've started work on the donations page it feels like I have to make a whole new site from scratch.

Is there any way of integrating buttons on my main wordpress site so the donate page looks like the rest of the site? I would also like to put buttons on the different 'Sponsor a Dog' pages. So for here example, I would like to add an 'Add to Cart' button under the information which will then take you to a shopping cart or paypal page:

http://www.thaianimalsanctuary.com/adop ... dog/elvis/

If this is not possible, can anyone give me any advice on what would be the easiest way to make the shop pages look like the rest of the site?

Thanks