Bug 22171: Fix shipping cost display on invoice

Bug 22171: Fix shipping cost display on invoice

When not 0 the shipping cost on the invoice would display with
a lot of numbers after the comma. This fixed it to display with
only 2.

To test:
- Make sure you have an invoice with shipping cost set (example: 5.00)
- Open the invoice for editing (invoice.pl)
- Verify the shipping cost displays with too many decimals (5.000000)
- Apply patch
- Verify now the shipping cost displays correctly (5.00)

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>