Well, OP did say purchasing with BTC and not purchasing BTC. In our country, whenever you make a purchase, you are also paying for a value-added tax, so there's that. It would be near-impossible for the IRS to keep track of your daily spendings, especially if you've mixed them first before sending them to purchase something you like. Taxing bitcoin is very vague when it comes to the tiniest of details; the government can't just tax you if you have bitcoins in your pocket, there needs to be some for of gain on your end that they know for them to get a tax cut, so how does that work in the long run if services like mixers exist?
Even if you are using mixed coins, you have to account for the merchant or payment processor you are interfacing with. Do they know your physical address? Email address, IP address? These details can be used to identify you. They can also be "leaked" because of insecure third party trackers/cookies. Between precise timestamping, the immutable blockchain and leaked transaction data/personally identifiable information, clustering attacks and blockchain analysis of a mixer's wallets can (up to a certain probability) be used to identify your originating wallet. And if you use a lightweight client like Electrum, you're also leaking wallet information to Electrum servers that can help to corroborate that connection (and also link your wallet to an identifiable IP address range).
I don't think the IRS necessarily cares to do this over low-value cryptocurrency transactions. But I think it's important to be aware that the government (in tandem with blockchain analysis companies) knows much more than people tend to think about your transactions. We shouldn't assume they don't have any means to deanonymize us. And we should be weary of exposing our real personal information, IP address, etc. when interacting with merchants and third party payment processors:
There are actually treasure troves of deanonymization tools available to governments which have been developed by Chainalysis and their competitors for years. They've got databases full of metadata like bloom filters, leaked tracker information and PII data, IP address data. For all we know, companies like Bitpay or other merchants are honeypots to identify our wallets via address clustering.
They're watching us and the problem is only going to get worse. If you think Bitcoin is "private" by default, then read this: arxiv.org/pdf/1708.04748.pdf