Copywriting is the art of persuading someone to buy your product, services etc... Great copywriting tends to focus on benefits in a clear compelling way... I'm not sure why you've put SEO in front of the term copywriting but I assume it relates to adding your keywords and variants of that into your copywriting as part of on-page SEO...
That's generally done implicitly and I personally do not worry about it... My SEO focus are on other pages which pull in traffic and then refer them onto the sales page...
Its pointless trying to get your sales page to rank for every keyword term... and further I think its best to focus all your efforts to writing the best sales copy which converts without ruining it by keyword stuffing etc...
