sphansen,
It depends on how you would like to implement this. While you cannot access a printer driver's options from PrintShop Mail Web, the choice of paper could be simulated using user-input fields, and then putting conditions in your PSM document to choose the paper size accordingly. Not sure if you'd be able to affect the pricicing though, as this is based on the chosen document, and not options within the document. An easy workaround, of course, would be to create different versions of document, and name them based on the paper's weight. This way, you could easily control the pricing for the various documents.
If you'd like to explore more possibilities, or discuss with someone about this, feel free to open a technical support issue. You can call 1-866-348-5863, or use the web page:
http://www.objectiflune.com/OL/en-CA/Support/ReportIssue/PrintShopMailSuite.aspxRegards,
Raphaël Lalonde Lefebvre