The opening line: prove you read the post
Most proposals open with "Hi, I am a [job title] with X years of experience." Clients have read this sentence thousands of times. They stop reading.
The opening line has one job: prove you read their specific post. Not proposals in general: this one, right now.
Reference something unique to their post: a phrase they used, a constraint they mentioned, a problem they described, a technology they named. The more specific, the better.
"You mentioned the checkout flow breaks on mobile for orders over $200. That's almost always a payment gateway session timeout, not a code issue."
Client thinks: this person understood my problem immediately.
"Hello, I am a skilled web developer with 7 years of experience in WordPress, WooCommerce, PHP, and JavaScript. I can help you with your project."
Client thinks: copy-paste. Next.