What are my options?
Not many choices here, I'm afraid. You can just send the bitcoin straight from your wallet to the seller's address. That's the most basic option, but might not be the fastest or most affordable. Bitcoin transaction fees can vary depending on the current network congestion, but generally youre looking at a few bucks at least and or you might want to try some other cryptocoins that are faster and less expensive than bitcoin like litecoin, dogecoin or stablecoins like USDT. But then you'd have to get your hands on those first, so that's not really doable unless you just happen to have some funds already parked in a crypto exchange account (and that comes with its own problems if you leave money there long-term).
I thought of Lightning but that is still going to cost me a lot in transaction fees when opening a channel. And since this is a one time purchase, it defeats its purpose because, well, might as well just directly purchase using btc.
That's right. For a single transaction, just paying directly with BTC is cheaper than using Lightning. LN is better if you're gonna make a bunch of little payments.