Firstly, I most highly commend you for your existing use of English. It's not an easy language to learn, and the item you've pointed to is really rather good - I only spotted a few small errors that matter, and I'm very picky. It's certainly much, much better than I ever got at French, German, Danish, Swedish, Bulgarian or Spanish. Overall, they didn't really detract from the item.

For many people it won't matter much, but for a few it definitely will. You need to decide how much it matters to you, and if you decide it matters, you need a copy-editor. You don't want someone to write the copy, you just want someone to tidy it up and remove the worst of the errors without changing the content much.

What contacts do you have? My circle of contacts would, for me, each copy-edit a few hundred words a fortnight. Depending how much I wrote, that would probably be enough without imposing on them too much. I'd also then buy them a beer, or cook them a meal every couple of weeks to say thank you.

There are also tools such as the Link Grammar. After spell checking, type what you want into that and see if you get one parse. If so, you're done. It's easy to download and install, and not so hard to automate.

That would be my recommendation.

PS: I'm a complete grammar-nazi and am probably much more severe than most people. Trust me - it was pretty good.