Well I'm not sure what country you live in but if your looking to buy bitcoins you can use
coinbase.com and if you add a credit card to file you can instant buy bitcoins and they are immediately added to your account for spending. If you're from Europe or other countries you could also wire money to bitstamp and buy bitcoins off the exchange there.
As far as you selling a product which requires the person to pay their cash, which then gets converted to bitcoin and then gets sent to you, all in 1 transaction...that does not exist, nor will it ever probably exist due to digital fiat transfers being reversible while bitcoins are not (no exchanger would want to take the risk and lose all that money).
Your best bet is to offer a discount for bitcoin purchases and then let the customers buy the bitcoins on their own and then send them to you for your good/service.