time travel - How could Walter send Peter the letter in the last episode?

THIS QUESTION CONTAINS SPOILERS FOR THOSE WHO HAVE NOT VIEWED THE LAST EPISODE

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In the last episode of Fringe, Peter finds a tape addressed to him, which was encased in amber. In this tape Walter says that if Peter is watching the tape, that means he probably got a weird letter from him and decided to go look for him the lab, thus finding such tape.

This tape was there since 2015, when Walter had already sent the letter and inoculated himself to go with Michael to the year 2167. But then something happened, and Walter had to amber the whole thing.
But this means that for the tape to have been recorded, the invasion must have taken place and Walter must have formulated his plan.

Fast forward to the end of the episode, once the timeline is reset, Peter gets said letter in 2015. However, unless I have misunderstood something, this is impossible.

From what I understand, the timeline goes something like this:

1. Peter and Olivia in the park with Etta.


2. Invasion of the Observers.


3. Etta disappears.


4. Walter elaborates a plan to prevent the invasion.


5. Walter understands he will have to sacrifice himself and go with Michael to 2167.



6. Walter records the tape and sends Peter the letter.


7. Walter was going to the future, but instead encased it all in amber, and then stays like that for 20 years.


8. All the events in Season 5 take place, ending with the timeline being reset.

Since resetting the timeline would modify things from point 2 onwards, how could Walter have sent Peter that letter? He could not have sent it (and recorded the tape) before the invasion took place.
Am I missing something or should we assume that the Walter in the future thought about this later and found someway to make the letter and the tape 'appear' in 2015 (in the reset timeline)?

Answer

I almost asked this same question, but I think I figured it out while trying to form the question. All it requires is good timing on the part of Walter.

• The plan was being put together from 2013 through 2015, if I recall correctly.



• The invasion (and the scenes with Etta in the park) happened in 2015, which is also when Walter ambered the lab and the team.


• September said that he didn't know what Walter did with the card, which implies he knew Walter had it while they were working on the plan before 2015.


• In the reset timeline, the card appeared to arrive on the day in 2015 when the invasion would have happened.


• Given Walter and Donald planning to exchange places, I'm not sure how accurate this is - but Walter said that the paradox that would erase him would occur at the time of the invasion in 2015.


• I believe earlier on in the season (but can't recall for sure) that someone said the invasion happened just when they were ready to enact the plan.

The intended timeline - and what apparently happened after the reset - was this:

In 2015, all (or, almost all) the pieces of the plan were together, and all they had to do was implement it. Walter knew this, and mailed the letter to Peter. A day or two later, Walter had vanished and Peter received the letter (seen in the last scene) and would later find the video.

Unfortunately, the invasion interrupted the last parts of the plan. Walter had sent the letter already at this point, so it was simply lost in the in post in the chaos during the invasion (This would explain why Peter didn't know about it, and Donald didn't know what happened to it). In a panic, the team quickly tries to get together the pieces to implement the plan, leading to them all being ambered and the events of Season 5.

There's a bit of fuzziness in that the plan had not been implemented by the time the Observers appeared in the invasion, so the paradox should have erased Walter before it was implemented, but he doesn't appear to have been entirely truthful about that anyway, given he was willing to trade places with Donald. It seems more likely the paradox would have occurred at a later point during the invasion when the plan was implemented - which would seem to resolve the inconsistency.