Ochūgen, the summer gift, thanks someone for their help, so it needs no return gift. If one is sent, an etiquette guide advises that it be of equal value or a little less. A return worth more than the gift received can be taken to mean: please send no more.
To decline openly, the guide suggests saying so in the thank-you letter, with words such as kongo wa okizukai naku, please do not trouble yourself in future.