Allow receipt cost to be 0 or no charge

[Simon Martin]

Sometimes we do not charge, so being able to set the final price to “0”or “no charge”

[Simon Martin]

The property is decimal... attribute sets the min value to 0.01... maybe allow 0 but handle it on the page?

[Simon Martin]

Fixes #53 the min value for receipt cost is 0.00

→ <<cset 8055fff51d68>>