Although at first glance, it looks quite expensive. Having to pay $10 monthly outright even with no charges feels kinda bad. I'm a huge fan of Amazon's free tiers to experiment with; Stripe's test mode is very nice for the same reason. And on top of this, the pricing being per-charge makes it far more expensive for businesses selling cheaper products ($5 subscriptions vs. $100 subscriptions).
Compare to Baremetrics' pricing plans which scales based on MRR: https://baremetrics.com/pricing -- At $50k MRR with $5 charge avg, Baremetrics costs $100/mo, Sigma costs 2.5x that. For $2.5 avg charge, Sigma costs double, Baremetrics remain the same.
They still sound like very reasonable prices though (and Stripe's fees are unfriendly to lower price points anyway).