Remove shipping for product addons (non physical)

My store has a product page where people can key in a specific amount to pay. If somebody enters $25.00, a $25.00 product is created and the customers place a $25.00 order.

This works if they paid for 2 day shipping, want to upgrade to overnight, and want to pay the difference? I don’t like collecting card payments over the phone so I do this.

I don’t need to collect shipping fees on this sort of product because it’s an extension of an ongoing order. My question is, is there a way via attributes that I can remove/hide shipping options at checkout and just collect billing details and payment? It seems like shipping is collected on every sort of sale?

Thanks for any tips.