Software for(computer/Mac Based) invoicing for retail store?

[PrancingHorses]

Wifey runs a clothing boutique and is currently using old fashioned numbered paper books invoicing for sales. Looking to get a software based e-system where she can log her sales and print out invoices, credit notes etc. What do you guys suggest? Been google-ing and it seems to be a minefield! She is VAT registered and has a credit card machine so if these can be integrated even better!

Has to be Mac or PC compatible as she does not want an Ipad based system. Thank you.