can i download it into my ipad and if i don't like it, delete it before i get an invoice without having to pay for it?
No. According to Apple, "all sales are final" > iTUNES STORE - TERMS AND CONDITIONS
Developers cannot offer trial versions through the Mac App Store. Follow the link in the Mac App Store to the developer's website to see if one is offered there. If you like the app and decide to buy it from the MAS, make sure that you delete the trial version before buying the paid version to avoid any app confusion.
The best way to delete apps is with an app such as the free AppCleaner.