Question regarding the Field agent sneak attack bonus

[SimonML]

The field agent (from Pathfinder society primer) can take sneak attack has an advance training.

There is no mention about taking this particular training multiple time, so; based on the description of the class; the player could not take it more than once.
So, it is a flat 1d6 that can't be improved? That's what the rules tend to suggest but it seems strange to me that there is no way to scale it up...

Same thing with the great casting, actually. The character can only improve his spells once?

[Cevah]

Field Agent Training wrote:

A field agent receives additional training that aids her in her fulfilling her duty to explore, report, and cooperate. She can't receive the same training more than once unless otherwise noted.

Sneak Attack +1d6 (Ex) wrote:

This ability is exactly like the rogue ability of the same name. If the field agent gains a sneak attack bonus from another source, the bonuses on damage stack. A field agent must be at least 3rd level before selecting this training.

It is a one-shot 1d6 by class training.

There is a feat that can get 1d6 also.

And it is "Greater Casting".

/cevah