gistfile1.txt

URL: https://targetxappdev1.na16.visual.force.com/apex/ProcessPayments?cost=100&cid=003j0000005XTPLAA4&retURL=/003j0000005XTPLAA4&redirectURL=/003j0000005XTPLAA4&item=Undergraduate%20Day&reason=Undergraduate%20Day&aid=003j0000005XTPLAA4&waiver=Yes&gateway=a0Tj0000001MISCEA4

Controller:

public with sharing class ProcessPayments_Con {
    public targetx_payment.cartHandlerClass handler{get;set;}
    public String retURL{get;set;}
    public String redirectURL{get;set;}
    public Decimal cost{get;set;}
    public String gateway{get;set;}
    public String reason{get;set;}
    public String item{get;set;}
 
    @TestVisible
    private String cid;

    public ProcessPayments_Con(){
        cid = getParameters('cid');
        gateway = getParameters('gateway');
        retURL = getParameters('retURL');
        redirectURL = getParameters('redirectURL');
        reason = getParameters('reason');
        cost = Decimal.valueOf(getParameters('cost'));
        item = getParameters('item');
        gateway = String.isBlank(getParameters('gateway')) ? 'gateway' : getParameters('gateway');
    }

    public void initCart(){
        handler = new targetx_payment.cartHandlerClass();
        handler.startCart();
        handler.setTransType(reason);
        handler.setTransReason(reason);
        handler.setTransContact(cid);
        handler.setTransCancel(retURL);
        handler.setTransRedirect(redirectURL);
        handler.addItem(item,cost,1,cid);
        //handler.setTransAccount(gateway);
    }

    @TestVisible
    private static String getParameters(String param){
        return ApexPages.currentPage().getParameters().get(param);
    }
}

VF:

                        <targetx_payment:paymentButton transID="{!handler.cartTransaction.id}"
                            totalCost="{!cost}"
                            CartHandler="{!handler}"
                            paymentReason="{!reason}"
                            payment_account="{!gateway}"
                            />