There is a well known rule that the gemara establishes in Nedarim 37a that one is not allowed to charge for the performance of a mitzvah such as teaching torah. This seems to create a difficulty in our gemara where the chachamim hold that one can charge for loading but not for unloading. How can one charge for loading, it is a d'oraysa mitzvah? Simply, this can be answered by saying that the default position is that one cannot charge for a mitzvah, but being that we have an extra pasuk (since we could learn out both loading and unloading from the pasuk of te'ina as the gemara explains), the extra pasuk is indicating that this breaks the regular rules and one can indeed charge for loading. However, Rashi complicates matters further because rashi (d.h. lomar licha) says that if the torah would not have written the mitzvah to unload, and we would have learned it from the mitzvah to load, we would have said that one MAY CHARGE for both loading and unloading. The maharsha points out that Rashi holds that the default without any extra pasuk is that one can charge for both prika and te'ina - the extra pasuk requires that one to prika free of charge. Therefore, the question would return, how can the default be that one can charge for a mitzvah, since in general one is not allowed to charge for a mitzvah? It can't be that the charge is only compensation for loss as we find by aveida, because then the gemara should have been clear about that.
A similar question can be asked when the gemara 32b suggests that one would be obligated to help unload an animal even when techincally the mitzvah of prika wouldn't apply i.e. the animal is overloaded or the owner isn't willing to help. The gemara explains that the requirement to help unload even without the specific mitzvah of prika would indicate that tza'ar ba'alei chaim is d'oraysa. Yet, the gemara says clearly that one would be allowed to charge for their assistance in unloading since they are not compelled by the mitzvah of prika, rather by the mitzvah of tza'ar ba'alei chaim. The question is, what is the difference whether the torah requires him to unload based on prika or based on tza'ar ba'alei chaim, it is a mitzvah of the torah that one cannot charge for? Perhaps the gemara understands that tza'ar ba'alei chaim doesn't place any specific demands on the individual. Rather, it is a torah recognized concept that would obligate one who witnesses a suffering animal to help alleviate the pain. It is only when the torah recognizes a specific action such as teaching torah, or healing the sick, do we say that one may not charge - מה אני בחנם אף אתם בחנם. But when one is compelled by a torah recognized concept, not by a specific mitzvah in the torah they have a right to charge.