Suggestion for long form like an invoice

Question

I am about to make a long form where users can select/enter data on a web interface directly. Something similar to filling up an invoice form on the web page. I wanted inputs on which process of the two mentioned below is easier for a user to fill up easily.

Option 1: Using a wizard and allowing user to make selection in two to three steps

[OR]

Option 2: Using a single form to fill in all the details

The details to be included are below:

1. Customer Details (name, address & contacts)
2. Invoice Details (Invoice#, Date, Due Date, Terms P.O#)
3. Product/Service Details (multiple products with rate, qty, discount, tax and amount)
4. Total Summary (subtotal, taxes, discount and TOTAL)
5. Terms & Conditions

Answer 1

The latter option gives the users an accurate idea of the scope of the task. Yes, option 1 may provide this using breadcrumbs, but that doesn't give FULL scope and IMO it's better to keep the users in the know as much as possible at all times.

Having said that you don't want an unapproachable form with too many fields, so subtitles, sections and even an accordion will help with that.

To give you an idea of what I mean (I've only done the first couple of form parts) the accordion on the left and sectioned form on the right:

^{download bmml source – Wireframes created with Balsamiq Mockups}

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Edit:

In a form on the system I'm working on sections could be minimised. In your case you could have a master section/category as listed above and then group inputs in to fieldsets. The groups could be collapsed meaning you can always sow mandatory fields (by making fieldsets containing mandatory fields expanded by default) yet still save space by hiding some areas of the form. Screenshot below.