How can I create conditions within the payment field items?

I have created a purchase order for calendars. The customer can choose to add an imprint of their contact info to the calendar to personalize it if they so choose.  However there is a one time imprint fee that needs to be charged for this personalization. I would like the imprint fee to be added to the product total cost (unit cost x quantity.) 

I also need a way to make it conditional as they can only order imprints for a order that has at least 50 units.

Is this possible?

Unfortunately, linking or applying conditional logic within items in the payment field is not possible. To achieve that and so you can have the "stock" option as you wish, you need to create a manual listing using the basic elements.

If you want to have a stock option then you need to use "ticket purchase widget" or "quantity gift registry widget". You can then add a form calculation widget to calculate the (quantity x price) of your item.

Guide: How-to-Perform-Form-Calculation-Using-a-Widget

You can also add the "imprint fee" you're referring to the calculation.

Once, you're done with the calculation, you can then pass the total value to the payment field. Kindly check the guide below for instruction/reference.

Guide: How-to-Pass-a-Calculation-to-a-Payment-Field