I recently posted a question about buying TGV tickets online. I finally found the easiest method compared to using SNCF or Voyages-SNCF.
One only needs to go to www.idtgv.com to find well laid out pages with simple format and FAQ's. The prices were the lowest available

Yes, that's the whole point of idtgv, but it doesn't cover all routes. When it covers what you need, it's great.

The site you mention is sometimes easier to use for those from North America but those from Australia often have their credit cards blocked on that site. In any event, any train listed on www.idtgv.com will also be listed, and tickets sold for the same price, on www.voyages-sncf.com or www.tgv-europe.com

When we tried those other web sites they would not accept our credit cards. We called the banks and they did not block the card. We think it was a processing error on the website since it went through smoothly on IDTGV.com

Does it not show any of the TGVs from CDG?

This changes from time to time but recently there were no iDTGV trans departing from CDG. Last year there were a dozen or so. For schedules, checking www.voyages-sncf.com will give you all of the options.

Am having problems ordering my SNCF tickets even though I ordered them online for several years and received them in the mail and never had a problem.

I tried a few times as I need tickets leaving from Burgos to Hendaye (I want to take the Spanish train to Hendaye, Fr. border and connect with TGV going into France. Each time I tried to order tickets for each route was told my credit card was rejected. Phoned credit card company to see if there was some change and was told there is no problem with my card. I was well aware of this as my balance is zero.

When I looked on Renfe site to get the timetable and prices I tried to order my ticket for first leg of journey but cannot see how to register for Renfe. I may have to resort to the ALSA bus if I cannot get a reservation. Any help much appreciated.