Orthography

Punctuation really trips me up sometimes. One issue for me is using punctuation with inverse ordering of the sentence for emphasis. Consider the following examples:

"If I were you, I would wait until next week to plan your flight."
"If you really want to go all out, I'd recommend you pick the more expensive model."
"Supposing I am your business partner, wouldn't you want me to tell you the truth?"

Note that all of the previous sentences can be reworded by taking the first phrase and putting it at the end of the sentence.

Is that comma necessary? Or is it an optional thing? This bugs me.

I'm guessing it is if it's separating two different clauses.

It's between necessary and optional, recommended, I suppose. You obviously can state your idea more clearly that way, having given your partner the time to predict and process what you're talking about, the rest will be just "yeah yeah I know this already".

It's been a long time since I've studied English, but I think what you are looking at is a prepositional phrase. Those do require commas.
Although, we are talking about a language full of if's, and's, or's, but's, and general bullshit.

I believe you would need a comma in all three examples because the initial clauses in all three sentences precede the main clauses.

If the positioning of the clauses were reversed, a comma wouldn't be needed due to the subordinate nature of the non-main (dependent) clauses. An exception could be when trying to emphasize a significant difference such as:

He was feeling very cold today, although it was over 25C outside.

In the end, it all comes down to writing style. Very few people follow the grammar rules exactly; however, as a rule of thumb, fewer commas are always better than excess.

Everytime there are isolated phrases in the reverse position, comma is required. Sometimes, it's not needed, but can be used for emphasyzing it.
In the examples you mentioned, they're necessary, i believe.

Just pointing out that in the third example you gave can have so many iterations and interpretations if you reversed the order, from what I've read.

0 "Supposing I am your business partner, wouldn't you want me to tell you the truth?"

- "Wouldn't you want me to tell you the truth, supposing I am your business partner?" if this were some sorta mystery game.
- "Wouldn't you want me to tell you the truth supposing I am your business partner?" I can't see this making sense as it is. It might look well but reading it out loud it just feels off.
- *obligatory third option*

Regarding the comma it's all about how you want it used, really, whether you want more or less of it or none at all.

I really prefer using commas and emphasizing meanings by reverting sentences.

It's a nice thing languages came up with.

Try asking on this site.