Often you'll find the word 'that' can be deleted from many sentences altogether without it affecting adversely the meaning of the sentence in question.
Many professional copywriters (myself included) one of our pet hates is using the word 'that' full stop. It is such an ugly word which all too commonly seems to be used far too much today by many people who ought to really know better.
As for other more general grammatical mistakes, I say not to worry about this too much depending on the sales / promotional piece you're writing for. Us copywriters, more especially long form direct response mail order letter writers, we all often break every grammar rule in the book to get the point across.
Perfect grammar isn't so important as actually driving home, communicating the benefits of the offer to the intended target market. Copywriting isn't actually about perfect grammar, what it is about is connecting emotionally with the person the offer is most suited to.
Indeed in some cases (not all), incorrect grammar can help you to come across with a little more believability. Everyone is human afterall and will make mistakes every now and then. Such is human nature. Completely understandable.
Of course, if you're in any doubt about the use of perfect grammar and you know this isn't your number one skill, simply employ the services of a proofreader to polish up the advertisement in question upon completion.