Hi... what a nice requirement! Right up our street. We're backend process/calculator specialists. We write calculators for everything... from simple international postal rate calculators to insurance calculators to complex bills-of-material estimation software. Your requirement looks fairly simple so this quote is a loss-leader... I'm more interested in providing you an ongoing relationship to maintain, develop, and keep your calculator(s) sharp and responsive. What we offer is continuity, with hosting and a full SLA. If you're interested in a longer-term support relationship, then we're your men! (Well, man and women - there are four of us in Datasmith, based near Wigan). For references... see the calculator on the British Royal Mail website, and look up 'The software toolcase'. Just ask for more info as you need it.
Tony