I had a problem and I could not find a solution. Hopefully someone else has had this problem and has a way to go with this.

I needed to have my shopping cart gather information on what laser type and wavelength of light the client was using prior to purchasing goggles. Well it seems unless you get a pro version of the K2Store you are not allowed the product option type of "input". You only get "dropdown" and "checkbox". The solution I found is a half azz one and it is the only way for now since I am not a coder...

go to yoursite/components/com_k2store/views/checkout/tmpl/checkout_shipping_payment.php

I dont know what line but you can "CTRL+F" and find "note", it will give you the "Customer Notes" and there you will see the "textarea" with the name "customer_notes"

Use that to change it to whatever you want to ask your client. It works. Also, right above that, there is a line that says "something echo" and it says K2STORE_Customer blah blah.... something like that. You can write anything in between the single quotes

<h3>
<?php echo JText::_('Change this to whatever you want'); ?>
</h3>