It's short for:
I would say so [as well].
say has it's normal meaning: to communicate.
In other words it means I agree with you.
There's two possible thing they could be agreeing with:
1 that the cowboy boots are f-in' awesome.
2 that there was lots of questionable clothing leftover.
It's likely that they mean the latter since they follow it up with a comment about the questionable nature of the tennis ball yellow piece. But they could be trying to remain intentionally ambiguous because they might think the boots should go as well.
Yes - You say that, but [some refutation or caveat] is idiomatically commonplace for many native speakers, in response to an assertion they disagree with.
It's nearly always used in a context where although the speaker doesn't fully agree with the preceding assertion, he recognizes that there's some truth in it.
In fact, you'll often hear You could say that, but..., which more explicitly acknowledges that the assertion is at least "credible", even if it's not completely accurate. Note that in this usage, you = one, anybody, not necessarily the person being addressed (i.e. - what it amounts to is something like It wouldn't be ridiculous for you/someone to say that, but it's not entirely true).
Also note that if the personal pronoun is stressed (You say that, but...), the implied meaning shifts significantly. This form almost always occurs in contexts where the speaker completely disagrees; he intends to dismiss your opinion on the grounds that you're ignorant or biased, and/or to cite other sources more authoritative than you to refute what you said.
Best Answer
Just to reiterate your context:
Here, the author wishes to share her notes with her readers in the upcoming days. However, she's very busy, and doesn't want to impose a deadline on herself. In this sense, her phrase essentially means that she'd like to promise us a date, but can't because of her other deadlines. Something like this:
You can then reduce this unnecessarily long phrase as necessary:
All of these have the same meaning.