Fatal error: Unsupported operand types in example.inc

When a flexicharge item is added to the total price of a purchase, the item name displays on the invoice, but with a $0 amount, resulting in the invoice total being incorrect (short by the Flexicharge amount).

e.g. Item 1 ($5) + Item 2 ($10) + Flexicharge item ($7-50)

Correct invoice amount should be: $22-50
Currently invoice totals as: $15

The Flexicharge item displays as a line item on the invoice, but with a $0 amount associated.