Is $5,350 to install artificial grass in backyard a fair price?

I recently hired a local artificial grass installer for my backyard in Ireland. The total cost is €4,800, and I’m paying in stages. So far, I’ve paid €3,000 in cash. The installer is a professional but didn’t provide an invoice or written agreement, only verbal confirmation and WhatsApp messages. I requested a change in the grass quality (didn’t like the one he brought), and we agreed on an additional €550 for better grass. I'm worried because after doing some reading, I'm seeing that a proper install usually includes excavation, crushed rock (804), compacted sharp sand, and proper drainage, none of which have been done here. I'm concerned about whether this installation will last in a typical Irish backyard with occasional rain and light to medium use, and the chances of sinking, unevenness, or drainage issues. I'm unsure if I should hold off on the final payment. The installer claims that since it's a new house, the soil is good and fresh, so no soil excavation is needed, and rock is not required as the sand is enough.