Good morning Clint A Ahill,
If understanding correctly you are wanting to integrate an XML interface from your distributor into your weebly store? If this is the case you will not be able to. HTML and CSS is the look of the site. XML would involve a database, so some SQL, and then some fetching scripts in either ASP or PHP. Weebly does not have this kind of functionality.
I can skin the design of your currently weebly store into a shopping cart application which would allow the interface customization. However, doing so would be far above the top limit of your project budget.
If not requesting an XML interface can you please clarify exactly what you are needing updated with the CSS or HTML for your Weebly store please?
-Sean