I just sent an email to a colleague about some of the extra little features in Active Purchase that I thought I would share with everyone else.
Here are some details about how you can manipulate Active Purchase by using QueryString parameters.
Show only a specific Product Group
default.aspx?apgroup=NAMEOFGROUP
Working Example…
http://www.activemodules.com/Produc...fault.aspx
Show only a specific Product
Default.aspx?approd=PRODSKU
Working example...
http://www.activemodules.com/Produc...fault.aspx
Add Item to an order
Default.aspx?apstep=X&apaction=additem&approdadd=PRODSKU
Working example…
http://www.activemodules.com/tabid/...fault.aspx
We have a few other options as well, but I will save those for the release. 
Thanks,
Will