You can do that in Snappy Checkout by creating separate products. Here's an example of where I do that with one of my products:
Pricing for Snappy Checkout is the same no matter how many tiers your item has. It's 2.9% plus $0.30 (for Stripe/PayPal) and 2% or $0.50 (whichever is less) for Snappy Checkout's fee.
If I did not answer your question, then please email me at Mike@SnappyCheckout.com with some more details.