Agreement with customers

We are a bio-tech company. We do have a Free Sample Scheme which provide free sample products (bio-reagents) to our customers. And the customers need to provide us an evaluation feedback or testing data after they used the products within 3 months. If the customers doesn't provide the evaluation feedback to us after they used the products within 3 months, we will charge them 60GBP for each sample he used. So we need a agreement for it.