In Primer, did they really need to do the time travelling themselves?

As they begin their exploitation of the stock market, both characters are afraid of bringing about any possible "paradox" situation. They sequester themselves in a hotel room. They do not communicate with anyone until they go "back" (second time through the same day), except to check the stock market. Only when the forgotten cell phone rings when they are not "back" do they allow themselves to play with that dynamic.

Therefore, any concept of sending a machine back in time to make the stock transaction for them would have been contrary to their (initial) conception of how to use this technology as safely as possible. Such a plan would force them to confront the Grandfather Paradox and related issues, which they did not have to do if they simply go back in time themselves (as long as they strictly follow their intended sequestration discipline).

In short, with the information they have at the beginning of the film, there is no proof that using a machine to do it for them would even work - maybe the machine makes them a bunch of money in a parallel universe, not in their own. The way in which both characters eventually seek to answer this question for themselves, lose trust in each other, and then turn on each other, makes the second half of the film so interesting (and confusing)!

Besides, there is the additional burden of designing a machine to extract itself from the time capsule mechanically after the correct delay, so that it does not "loop" forward and backward through time, only to be removed at the time the capsule is powered off.