Just wanted to share a tip we discovered recently when looking for the best price for a one day return trip from BA to Colonia and back again.
From the office, it's not possible for tourists to buy tickets in pesos but we paid US$54 each for same day return (Monday to Friday), leaving 8.45 am returning 9.45pm. Vs the online price of $700 pesos (which at time of writing converted to around $110 USD).
Seacat prices online were cheaper than any other carrier (Colonia Express & Buquebus). Not sure if these companies also offer cheaper tickets if purchased in the stores.
We bought our tickets the week before travel.
Any questions let me know. Hope this helps others.