Hi there, As per your post i can help you to design and develop the calculator in swift along with the features you need and assure you for the long term support.
I am experienced mobile app programmer, having 6+ years experienced in programming similar kind of Apps (Dating, Taxi, GPS tracking, Parking) and many others.
-Online payment
-Ordering online
-location tracking and other features.
I hope to hear from you soon.
Thank you!