Welcome to Shopify Community. My name is Olivia and I work here.
Thanks for sharing your question with us. It sounds as though you are hoping to generate a random number for the handling fee for each transaction? As the flat amount section of handling fees is set to be a specific amount, I would recommend that you take advantage of the percentage markup as seen in your screenshot.
If you choose the percentage option instead, Shopify will automatically calculate x % to be added to the calculated rate. If you use Shopify Shipping to pull live rates, this would result in a unique handling fee addition to each transaction.
Note: If you include both an additional flat fee and a percentage markup, then the percentage is calculated before the flat fee is added. For example, a $5.00 rate with 50% markup and a $1.00 flat fee costs the customer $8.50, not $9.00.
I trust this would do the trick, but you can always look into an app like Order and Product fees to diversify your price points too.
Please let me know if I have missed or misunderstood anything.
I look forward to your reply,