How do you set up a SaaS as a solopreneur?

Hard to be very detailed here without knowing a lot about what you are doing.. However here are some ideas.

1. USA - Stripe for payments and managing subscriptions. They have numerous tools for doing so. Their subscription handling is pretty good at the moment and useful for people who want to cancel etc. The handled processing, fraud etc. Pretty simple to setup with your saas.

2. Talk with an accountant or lawyer for your specifics regarding the state. They differ wildly so you want to make sure you are prepped and handling that from the beginning. Any good accountant should take about an hour to break this all down for you. Worth the $$.

Best of luck.

Just use a pret-a-porte payment system, lile Stripe, Square, Paddle, Clover etc. They will handle your plans and recurring payments for you.

While I agree that Stripe is great for payments, Stripe Subscriptions and Stripe Billing were never really built specifically for SaaS. The problem seems easy at first when you have just one plan at $100/month, you'll quickly have many more plans, all kind of discounts and special deals, custom "enterprise" plans for your larger customers and design partners, while having to grandfather your existing customer base and also allow easy upgrades/downgrades etc. A lot of SaaS pricing these days is built around tiers with certain usage limits on individual features, so you'll need to built that as well. And if you are working on an application with a fixed underlying cost for certain features (like genai tokens), you'll also have to implement metering. None of this is really part of your core product and yet most SaaS startups spent a lot of time building and maintaining (billing is hard!) the same functionality over and over again.

There are a few new modern players (disclaimer - I'm the CTO of one of them [0]) in the SaaS subscription mamagement and adjacent usage billing space now and I would highly recommend using such a solution while still relying on Stripe for just the payment processing part. It will save you a lot of technical headaches down the road and even should allow you to get started quicker (for example, we provide you with embedded pricing pages, quoting and sign up links, feature entitlements per customer, full metering support and much more out of the box) and iterate over your pricing faster as you grow.

[0] https://wingback.com

Avalara (https://www.avalara.com/) does tax calculations for SaaS - you give it a list of prices, the address of your customer, and the type of good/service each item is, and it tells you the correct amount of tax to charge :)

1. and 2. Just use Paddle.

Note that you still need a legal entity to receive your income and probably an accountant to do your taxes.

Stripe, Paddle and Lemonsqueezy are good choices.

Keep it simple in the beginning just set up subscriptions/billing and a few basic plans to get users signing up and working out the flow since it sounds like your first time.

Lemonsqueezy and Paddle handles taxes out of the box (Stripe is working on a tax solution). It sounds like most jurisdictions have thresholds you have to break before you have to pay sales tax. It varies greatly so it's probably worth a little extra cost to have that covered from the start. But early on I'm not sure it's completely necessary.

What framework are you building on?

Here are SaaS in a box billing/subscription tools for Rails and Laravel https://bullettrain.co/ https://spark.laravel.com/

Good luck.