I'm looking to recreate, in Salesforce, a form that some of our officers use for processing payments.  The part of the form called "Amount Due" requires several formulas, which I'm not sure how to create.  As I'm new to Salesforce and creating formulas, any help would be appreciated!

The form looks like this, with the items in bold (Previous Balance, Credit,Total Due, and Remaining Balance) representing what I need to figure out formulas for:

Subtotal: (Greg Rohman helped me with this, and is an amount calculated, in US dollars, by multiplying # of documents by price per document)

Delivery Charges: (This is a flat fee and is therefore a picklist for either 0 or $15)

Previous Balance: (= Remaining Balance, if positive, from the last time this company did business with us)

Credit: (= Remaining Balance, if negative, from the last time this company did business with us)

Total Due: (= Subtotal + Delivery Charges + Previous Balance - Credit)

Total Paid: (This is manually enterered by our officers, in dollars)

Remaining Balance: (= Total Due - Total Paid; if positive, this becomes the new Previous Balance for the company on their next invoice; if negative, this becomes the new credit for the company on their next invoice)

Would you have any ideas about how to do this?

I'm a new Salesforce user, and I just started using Trailhead. I am having trouble navigating my way through because the Trailhead Playground I am using doesn't match the instructions. The layout, menus, and options are all different. Can anyone help me figure out why this could be?