Where is the best place to add the add voucher code on my e-commerce checkout process?
Shopping Cart Page
Normally I saw it on shopping cart page but the user will think on payment method and delivery on the checkout page.
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Assuming "add voucher code" is where the user enters a code to receive a discount...
I would consider adding it in both places. If it is on the cart, the user can easily add it and see their total cost even before they are ready to check out. If they did not enter a code on the cart page, ask for it again in the checkout flow. If they have already done added one, you can just display it again in the checkout flow.
It's a tricky one. E-commerce users tend to be price-sensitive. If you show them the voucher field, they will have a feeling that they can save. They google for the code, try several and if none worked for them, feel frustrated and could leave your website with a bad feeling and without a purchase. Moreover, then can get distracted by something and never return to your checkout.
I'd recommend to use the voucher code only in the checkout form. If you are not sure the user will find the code somewhere, I'd hide the voucher field under the link "I have a voucher code" so the user is not teased with the possible discount offer.