Test transactions

Hi,
I have been trying to test the plugin.
I am using:
Note: For test transactions, please use Merchant ID : ABC0001 and Password: abc123

And using…

In the test environment, please use the following credit card to process test transactions.

Card Number: 4444 3333 2222 1111
Expiry date: Any future dates
CVV: any 3 digit number`

And have been:

In the test environment, if the payment amount ends in 00, 08, 11, or 77 (e.g. $1.08, $1.00) the transaction will be approved otherwise it will be declined. You can use this test environment feature to simulate approved and declined transactions.

I am getting the following message when I try to checkout a transaction for $543.77

“Payment can not be processed (Reserved for Private Use)”

And when I try to checkout a transaction for $489.49 I get the following message:

Payment can not be processed (Unknown response code/Unable to read response text)

Why am I getting these messages and what messages should I be getting in test mode?
Thanks