Let me suggest a few more things. First, do not use a round number, like $400. Rather use a more precise value that reflects the idea that you have figured out the cost very closely and it is the bottom line. For example, use a value of $403.77 each. Similarly for travel...state that you will charge a professional fee of $80 per hour for assembly plus actual travel expenses, estimated to be $ 780. Your fee will hardly cover the parking for you vehicle. In the future, suggest that a local helper would save time and money.
Second, use a technical name for the item that reflects its uniqueness. Perhaps include "extra strength" and environmentally pleasing. Point out that you or someone else can easily maintain and repair if they are abused. Mention that they are well design to avoid racking, carry heavy loads, etc. there is no question that your cost reflects quality that does not exist in the units they are going to get....your quality is deserving for a quality gallery.
Finally, as a general rule, anyone who buys on price, rather than quality and delivery, is likely to be the same person who complains.
Overall, it is clear that you have an awesome product and service, but it sounds like you do not have a representative that told the customer all the benefits of having your products and services. You have to "toot your own horn" as no one else will do it.