We've known about Sony's and Rockstar's special partnership since before the game launched back in October last year, but it's still taking on new forms.

Recently announced on the official PlayStation blog are a new series of monthly benefits that players who chose the Sony console will enjoy on the frontier.
Every month from now on, PS4 players will receive a gift package which can be retrieved at the post office or at your camp's lockbox, if you have the upgrade. These packages will contain different items each month, and the first one - which can be picked up starting tomorrow - comes with three treasure maps. You can expect gifts of similar magnitude in the future, so don't expect a high-end horse to be mailed to you by Sony.
You'll need to have played the game at some point prior to the 8th of June to be eligible, so if you only happened to buy and play the game today you won't be getting the special gift. The package will be mailed to players within 24 hours of their first play session within the timeframe. This would indicate that you need to play in the given month to be eligible, so if you put down Red Dead Online for a few months and return, you won't log in with several packages waiting.
Though having a PlayStation Plus subscription is usually necessary to play Red Dead Online, due to certain promos which allow temporary access to players without subscriptions, we should point out that only PS Plus accounts will get these benefits.
Treasure Maps, which are this month's bonus, are a great way to earn some easy cash. You need to open up the map from your satchel, which will then mark an exploration zone on your map. Ride out to the area and activate Eagle Eye to look for the hidden goods. Once you dig up the treasure, you might find gold, RDO$, consumables or crafting pamphlets.
Next month's benefits will be different, so stay tuned for more news on that.