Automatic transfer switches are not required for portable generators. An inexpensive interlock kit will serve the purpose just fine. The one I will provide a link to is UL approved. However, you can find them for less money.
No need for an automatic one for sure, but some kind of interlocking emchanical switch will be required to prevent the generator from backfeeding the grid under all circumstances.
The other method mentioned, basically a permanent extension cord inside the wall, is indeed safe but not very user friendly. It's most definitely cheap though.
