Hello, I have seen your request, and I think I am the right person to carry out the project.
I worked with joomla 1.5 and virtuemart for several years, so I have the required experience, to be honest, I have not worked too much with joomgallery, but given the description of the project, the most important work must be done for virtuemart, since it is only necessary have an identification of each image of joomgallery to be able to obtain the thumbnail, because the most important thing is the invoice that must be developed for virtuemart.
I think the best way to proceed is to create a module for virtuemart that obtains an identifier of the image that presents joomgallery according to the page in which the user is located, in this way the required information will be stored to create a value to pay for the license of the image batch. However, to be sure, I intend to analyze your installation of joomla and its components well before giving you a final verdict, that is the first milestone you can see in the bid that I have presented.
I still have some questions concerning the functionality you want, such as the exact location from which the user can add items to the car, or the way in which the transaction should be prompted.
In any case, I will be attentive to your questions and suggestions.
I hope to work with you on this project.
Have a nice day.