Whether or not to include over the before last year is largely just a stylistic choice, but...
If you do include over the, this strongly implies that multiple investments were made on several occasions, or continuously. If you don't include it, that interpretation is still possible, but it's also possible the entire investment took place on a single occasion.
The difference between using (Past1) has caused or (Present) is causing here is exactly what you'd expect - use the latter if the process is still ongoing, otherwise use the former.
The logic of the real-world situation in the specific example may make it difficult to decide whether you want to describe the "increase in sales" as an ongoing process, so it might help to consider a more clear-cut alternative...
1: The covid lockdown is causing great distress
2: The covid lockdown has caused great distress
...where #1 strongly implies the lockdown is still in force (and people are being distressed right now), but #2 could be used during or after a lockdown (possibly even "after the distress", if that distress is still "relevant" to the current situation).
1 In case this is confusing, note that Present Perfect (has caused) is used to reference things which happened in the past, but which are either still happening or still relevant at time of speaking. But as pointed out above, it might be a matter of opinion whether "causing an increase in sales" is "relevant" and/or "ongoing" in OP's example.