CO has a very unique sales tax system: every county, city, and special district can set its own rates, and define taxable (or nontaxable) product categories, and determine the taxability of of those product categories, and even apply special (formulaic) discounts for complying with the tax regime. Plus, a number of tax jurisdictions overlap. And this is the simplified system that the state implemented in 2019 to make things easier for remote sellers in a post-Wayfair world.

And that doesn't even include the roughly 70 home-rule cities which administer their sales tax separately from the state's "simplified" tax system.

For filing purposes, each CO "tax location" is treated as a separate return by Avalara and other sales tax services providers, so the costs can add up very quickly for remote sellers using these services for sales tax reporting compliance: Denver alone has more than two dozen separate "tax locations." Consequently, at my company we use Avalara to compute sales tax for remote sales to CO but file the CO returns ourselves and save several thousand $$$ each month.

Luckily, due to Wayfair nexus requirements, CO sales tax compliance only kicks in at 100k or more in gross sales to CO (not including sales to home-rule cities). Because each home-rule city has its own sale tax system, the nexus threshold is independent for each home rule city; the state has advised these cities to set a threshold of $100k to avoid nexus-related litigation that could result in a bright-line rule setting an undesirably high threshold for nexus.

Because Colorado has the most garbage sales tax regime in probably the world.