Hello, Ben. I can see the benefit of automatically adding a series of numbers in the Amount field when entering a bill. I'll share some information on how you can do this below.
I agree with Rainflurry that you can enter the equation you want to solve on your bill's Amount field and press the Tab key or click anywhere outside the said field to get the total of the numbers you've added. Please see the screenshot below for your reference.

On the other hand, if you have other preference in adding a series of numbers in the Amount field when entering a bill (for example, use the Enter key instead of the Tab key), I would encourage you to send suggestions or product recommendations. We'll take them as opportunities to improve the various features of our products.
Additionally, after paying your bills, record your payments in QuickBooks to keep your books accurate. You may want to check out this article to guide you through the process: Enter and manage bills and bill payments in QuickBooks Online.
I'm all ears if you have other feature concerns or questions about managing bills and expenses in QuickBooks Online (QBO). You can drop your comment below, and I'll gladly help.