Build an app

I am looking to get an app developed that can be use on Apple and Android. I currently sell fertilisers for lawns and I would like to develop an app that can do the following things;
- calendar reminders for product applications. So if someone buys a particular product I can load their profile so it reminds them when to apply a product.
- they can also select that product and it will take them to our website for purchasing of that product
- ability to push notifications for suggested products to apply
My site is greenlawnsupplies.com.au (shopify site)
I
Am unsure of costs so please ignore the budget for now