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Let us Assume, some products are purchased by the customer.You can get the Order details such as Total no of products,product cost,billing details etc ...in your website.Before redirection to the payment page get the "selected" product details,customer profile details and store it in db.
Then you are redirected to Payment Gateway service provider page.There customer enter the Banking Details and payment get processed.
Note:
Don't Store Customer Bank details in your Database(It's not recommended)
Once the payment is successful you can get the Response code,Transaction status,Billed amount etc..from the provider.Then perform the Database operation.If Transaction is succesful then update the order status in db and show it in User profile.
Contact the Payment gateway service provider to get more details
Start Here:
Introduction to PayPal for C# - ASP.NET developers[
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Use of the PayPal payment system in ASP.NET[
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PayPal Gateway Integration in ASP.NET[
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