Mesoamericans did not use pack animals to transport goods. how do you think overland trade would have been affected if mesoamericans did have pack animals?

Pack animals give an opportunity to carry more things, and they also mean that people don't have to carry things on their backs: this leaves people free to do other activities.

Because of this, pack animals would mean that there would be more trade and more exchange between the cultures, for example  between the Mayas and the Nahuas. More goods would also have been traded.

but it could also mean that there would be more achievements of other sorts: done by people who were busy carrying and that could not do them.

Pack animals were actually available in South America: the llama.