In that case, the only things that can go wrong are:
1. You don't have same item number in E-junkie and TrialPay
2. Currency in E-junkie admin for your product is not USD
3. You have not entered GET Parameters/POST Body specified in the documentation
4. You've not set
https://www.e-junkie.com/ecom/tp_plug.php as Notification Script URL in TrialPay