In the scenario, you gave here that correct look on it would that you would accept the gift and not be accepting of the gift. To accept something you are agreeing to take something and to be accepting of something you are more understanding the situation you are in and you are becoming okay with the situation. So you accept gifts and you are accepting of people and thoughts. I hope this helps.

You should accept this unless you want otherwise. Note that Progressive tenses of that kind often can be used in colloquial speech. For example, "I'm gonna be doing my homework," "I really should be going," etc.

Accept is something you do once, while be accepting is something you go on doing. Actually, in the example you gave, either is correct but "Should I accept this?" is more standard usage. If you say "Should I be accepting this?", probably you are in the process of taking it with your hands and are questioning whether you should be doing what you are already doing.