If the payment gateway assembly is not found by Kentico CMS, your custom payment gateway form will not be loaded.

Following properties of your custom payment gateway are case sensitive, so you need to be sure that you specify them correctly (according to Developing custom gateways) :

Payment gateway assembly name:
Payment gateway class name:

You also need to ensure that you are using correct dll files with your custom payment gateway dll file. In other words, if you have built your dll file in your development environment and you are delivering the custom payment gateway dll file to your live web site, please copy all of the other Kentico CMS dll files together.

Also please check Kentico CMS Event log (CMS Site Manager -> Administration -> Event log) for possible error messages which might lead you to the cause of this issue.

You can set the Currency formatting string in the CMS Desk -> Tools -> E-commerce -> Configuration -> Currencies -> <your currency>. This property uses Standard Numeric Format Strings. In this example, enter following string: ${0:F3}.
See Also:
Standard Numeric Format Strings

First of all, please check if the payment method is enabled in: CMSDesk -> Tools-> E-commerce -> Configuration -> Payment methods -> <edit> -> Enabled: checked
Then the payment method needs to be assigned to appropriate shipping options. Please check if your payment method is enabled in: CMSDesk -> Tools-> E-commerce -> Configuration -> Shipping options -> <edit> -> Payment methods tab

This problem is partially caused by PayPal. PayPal somehow remembers all of the OrderIDs, so in case you performed some test orders and you have deleted them after and tried to create a new order it might have the same OrderID that has been previously marked in PayPal as "paid".

We suggest you to go to the Kentico CMS database -> COM_Order table and check your 'OrderID' column, eventually to increase next OrderID to some value you are sure has never been used before.

Or, it seems that the email you should use in Kentico for the Paypal integration is a seller account you have to create under Paypal and NOT the email for the entire sandbox.

This is a matter of what code is used in appropriate transformation.
You can edit transformation used for wishlist web part, find following line:

<%# EcommerceFunctions.GetFormatedPrice(Eval("SKUPrice"), Eval("SKUDepartmentID"))%>

And replace it by this one:

<%# EcommerceFunctions.GetFormatedPrice(Eval("SKUPrice"), Eval("SKUDepartmentID"), Eval("SKUID"))%>

There are two invoices - in fact they are the same but used separately. One is used only on the site after the purchase is made and the second one is used in the e-mail template. Their locations are following:

1) CMS Desk -> Tools -> E-commerce -> Configuration tab -> Invoice (devnet.kentico.com/docs/ecommerceguide/invoice.htm)
2) Site Manager -> Development -> E-mail templates -> E-commerce - Order notification to customer template

You can modify ProductOptionSelector control.
Please find a description how to do so below.
1) Open <your project>\App_Code\CMSModules\Ecommerce\ProductOptionSelector.cs file
2) Find line about 431 - item.Text += " (" + preffix + formattedPrice + ")"; here you can add the number of available items.
3) You can get it using this piece of code:
int items = ValidationHelper.GetInteger(row["SKUAvailableItems"], 0);

You can apply tax to shipping in custom CalculateShipping method (please see Overview and Using Custom Providers sections in E-commerce guide for more details about how to implement custom provider), that is used every time the shipping value is required, and return shipping including tax from this method. If you want also to display the tax in shopping cart content/tax table you can use custom EvaluateShoppingCartContent method - you can store calculated tax into cart.ShoppingCartCustomData hashtable in CalculateShipping method and then you can acquire it in EvaluateShoppingCartContent method in order to to display it in tax table.

We support the Website Payments Standard at PayPal. For more information please see "Website Payment Standard: Overview".

This happens mostly when you are copying a text from some text editor, like Word. In the transformation which is used to display the product details, the default setting for the description field is set to be HTML enconded:

<td><%# Eval("SKUDescription", true) %></td>

Please change it to:

<td><%# Eval("SKUDescription") %></td>

Text: The subject of the e-mail is a resource string. You can find the key value in the ~\CMSResources\CMS.resx file. In this case it should be "ordernotification.customersubject" and "ordernotification.administratorsubject".

You can change the values for these key directly in the resource file. The "{0}" is a macro and the orderID is displayed instead.
Other way is to add these keys to the Site Manager -> Development -> UI Cultures -> click to edit En culture -> New string. Add the key value and the text you want to have. This will ensure that you will be able to easily translate the subjects in other languages.