This all sounds convoluted and awkward.
You say you have an iPad. I imagine that might be easier to design for using FM Go and could server the purposes of both the laptop and the iPhone with a sim card and 3G.
Printing the invoice might be done with a mobile printer or achieved by emailing the PDF or uploading it to the cloud so they can download it from an emailed link.
The Barcode thing is a bit of a dilemma... but there are those square barcody things which I have a app for on my iPhone. Are these suitable, anyone?
If you were to accept say PayPal (who also take care of the Credit Cards) you could have it in a webviewer and have the person enter their details by handing over the iPad. You could also get them to sign on the dotted line and capture a signature (probably temporarily is best for signing a contract then deleting the image once captured in the PDF which would have the ability to export or change the document turned off).
You could do the whole thing as a web site also if your backend were FMS.
I just think there are too many steps...
LOL. "server the purposes" should read "serve the purposes".
I can't edit the post on my iPad. Wrong Device it tells me.
Also, I sorta ignored the issue of having no 3G or wifi network. I know cellular data can be expensive but for a short on-then-off-again task it amy just have to do. Surely it is better than shunting data this way and that....
We built a point of sale / inventory control system entirely in Filemaker. We purchased Symbol LS2208 barcode usb scanners and since we are running on a computer, we use the 360Works Plastic plugin to handle credit cards. It works with PayPal very nicely. But, if you wanted to go the plugin route, you would be limited to the use of your laptop, since FM-Go dosen't support plugins.