Hey @afifaakhtar, not sure if you've found a solution but I'd like to share my perspective.
You mentioned that you do not want to use any app and that you'd prefer to build an app. I understand this reply comes 2 years late but I hope you were able to build an app with the functionality that you mentioned.
It isn't an easy feat building an app that would route your orders according to such conditions, and if you increase the number of fulfillment centers it gets even more complicated. On top of order routing, how are you handling inventory syncing or sending your tracking information back to your customer at this point?
If you are still in limbo trying to resolve this issue 2 years later, maybe consider using an app like Duoplane. It'll solve your problem, and provide you with even more functions such as inventory syncing, shipping feed automation, analytics, accounting integrations, and more. It is a multi-vendor dropshipping automation app that will automate a huge part of your business. Best of all, the customer support is excellent, and you can get a free trial before making any commitment. Do your research to see what best suits your needs. Don't just take my word for it - check out Duoplane on the Shopify app store and read the reviews.
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