There are a lot of problems with this plugin. I purchased the pro version and then you refused to refund me after I asked for one that was within the 7 days specified.
The main problem is that you can easily add more than the maximum number of allowed products. Your solution to the maximum allowed is the hide the button when the maximum is reached but you use ajax this does not work. You need to change it so there is a message on the checkout saying there are too many samples and also disable the checkout button.
If you can fix this it might be an okay product.
Also, it would be good if once the maximum number has been reached we can we can have a message instead of the button just dissapearing.