Hello,
Thanks for your invitation.
We have quite good experience in Shopify Customization as well as development of custom Shopify Apps. However we haven't built any Shopify App which is on store already. Although this should not be a challenge, I thought it is important to share the same.
Let me know if you would be ok with this. I will be interested to know further about the requirement so that can analyze the same and quote the same.
Regards,
Chirag