Call method from another activity through JsInterface

Question

I'm trying to develop a way to fire inapp purchase from a JSInterface.

I have two classes.

The first one is MainActivity, in which i implemented inAppBilling with Iabhelper. This activity contains the method i use to purchase an item :

protected void purchaseItem(String sku_item) {
    final String sku = sku_item;
    IabHelper.OnIabPurchaseFinishedListener mPurchaseFinishedListener = new IabHelper.OnIabPurchaseFinishedListener() {
        public void onIabPurchaseFinished(IabResult result, Purchase purchase){
            if (result.isFailure()) {
                Log.e("PURCHASE","Error : "+ sku);
                return;
            }      
            else if (purchase.getSku().equals(sku)) {
                Log.e("PURCHASE","OK : " + sku);
            }               
        }
    };
    mHelper.launchPurchaseFlow(this, sku, 10001, mPurchaseFinishedListener, "mypurchasetoken");
}

The second one is a JsInterface which grabs the item sku from the user click in the webview :

public class jsinterface {
Context mContext;

/** Instantiate the interface and set the context */
jsinterface(Context c) { mContext = c; }    

@JavascriptInterface
public void purchaseSku(String sku) {        
    Log.e("JSInterface","Purchase SKU : " + sku);        
    MainActivity cls2= new MainActivity();
    cls2.purchaseItem(sku);
}
}

My tests show that :

purchaseItem() from MainActivity is ok and functional
purchaseSku() from JsInterface is catching the right sku from user's click
But purchaseSku() seems to fail calling purchaseItem() from MainActivity... Nothing happens. Do you have any idea of what i'm doing wrong and how i can correctly fire purchaseItem() from purchaseSku() ?