This was never answered

Hm.. i want to say you can buy them now and it doesn't matter. I'm not 100% sure, but that seems to be how it was worded.

The post actually already answers that question.

$50k: Pack Bonuses - All previously purchased backpacks (Adventurer's Pack, Founder's Pack, & Merchant Pack) will each have additional goodies added to them for any that were sold (a la carte or as part of a package) before June 1st, 2018

While the original post from last year that Violet quoted stated it was time limited, the Pack Bonus boon actually awarded a few weeks ago does not seem to have that stipulation included.

So the OPs question, which has been asked before, is did they drop the stipulation when it was awarded, or is it "silent"? If the former, then I see lots of people buying extras. If the later, then it's something that needs to be spelled out to avoid confusion.

The original post would be the clarifying point on it.

To elaborate on Violet's answer: even if it's not clear from within the inventory on our end, as the pack bonus is a single separate item from the backpacks, it's still information that SBS should have on their end. This means that once the time comes to convert website inventory to game inventory then they should still be able to convert the relevant backpacks to its "plus" variation easily enough based on the database. The same goes for the pet tokens valid for phoenixes as another example.

It's quite possible the pack bonus item was just added to our inventory to let us know we're eligible for at least one such pack bonus and is completely irrelevant on their end. Or in other words, pack bonuses will most likely be applied according to date of acquirement as specified according to the original post.