My name is Alex, I'm a guru here at Shopify!
Congratulations on your 100th order, that's fantastic!
It's not currently possible to set a payment method available to certain products only; if you've added COD by creating a custom payment method, then it appears on all checkouts going through your store. When creating payment methods in this way, you do have the option to provide instructions; you could, for example, create text there saying "COD is available for PRODUCT 1, PRODUCT 2, or PRODUCT 3 only - if you select COD for other products your order will be cancelled", or if you have a fairly large amount of products you'd like to offer COD on, you could organize them into a collection and specify that COD is accepted only for those products.
I may have a workaround that can help, however - I notice that your store is selling in Rs, are you located in India? We have an app developed for stores that are based in India and sell in Rs called Advanced Cash on Delivery that can help! While it still isn't possible to restrict that via individual products, using this app does give you more control over what customers can order using COD - you can set restrictions based on the postal codes of your customers, or specify a price range that COD is available for. The documentation on that app is right over here, and might provide the solution you were looking for!
Hope that's helped! Please feel free to give us a call or start a live chat at any time, we're open 24/7 for your convenience and always happy to assist!
Alex | Shopify Guru
Yah I'm from india. I am using advanced cash on delivery app from past few months and that is helping me on limiting COD to postal code.
Writing instructions in description will be a better idea for time being. But if I cancel orders, then this will fetch me bad reviews on long run. Can you suggest me any other app that can help me.
As Alex stated, this is normally impossible via any standard methods such as apps or the Shopify admin. However, it is possible and we have developed a solution for several stores. Based on your requirements the development time would be very short - no more than 2 or 3 days to get this done, and the cost would be low. Would you like us to contact directly you to discuss?
This can be done only in Shopify Plus (Gold in case of India) plan. We have done that for a couple of clients. This requires a small coding/scipts to be implemented. We also have an App which can get the COD SMS OTP confirmation as well. Please let us know if you would be interested. You can write to us at firstname.lastname@example.org