Anybody with a security clearance (whether working for the government or a contractor) has to periodically renew said clearance. In theory, you can't be compelled to answer questions by the government. In practice, if you wish to keep your job you're going to need that renewal and there's no guarantee they're going to approve that renewal. This is especially likely if they see stuff they don't like or if you're not forthcoming with them. Most people in that position will make a conscious decision not to engage in any practices that could look suspicious, even if there's a perfectly innocent reason for the behavior.
Engaging in crypto transactions with a Chinese entity would be an example of that, as it could imply a financial relationship with a foreign government that you were trying to obscure, which would definitely be something the US government would be actively looking for. It doesn't matter if it's only a few hundred dollars. For all they know, you've got much bigger transactions taking place that their system didn't flag, so they're likely to want to dig into the nature of the transactions and take a much closer look at the rest of your financial transactions.
Now, let's imagine this is taking place 5 years from now, you've completely forgotten about these purchases or who the company was, you answer a question wrong, and now it looks like you're trying to hide something from the investigator. You say "oops," and explain the mistake. Now you're getting asked a dozen other questions you wouldn't have otherwise been asked. Even worse, perhaps the crypto wallet you sent to was mistakenly flagged as being affiliated with a terrorist organization. The investigator is sympathetic to you, but your telegram (or whatever) messages related to the order were lost and are no longer accessible to you. They deny your renewal. Your boss terminates your contract since your job requires the security clearance. You lose your house and cushy lifestyle. You say to yourself that there's a silver lining in this: At least now that you can't afford food, you'll finally lose that last 50 pounds, but despite being homeless, the EBT program ensures you can still afford food and you gain 50 pounds from stress eating instead.
I'm being hyperbolic here and this is an overly dramatic description of events, but this is the thought process that anyone with a security clearance will go through.