What's the latest we can close sales before the race starts?

You control when sales end by selecting “hide” for the buy button.

You can sell right up to your start time but legally you must accept postal entries 48 hours after you close paid sales to allow for postage time. You can add postal entries after the race has started as balloons in last place move faster until they catch up with the pack.

It’s very unlikely you will get any postal entries:

In March 2021 34,000 people used the virtual balloon race at balloonrace.com and the PDF free entry form was exported 19 times. Most races receive zero free entries because the balloons/ducks are usually sold for £3 each (an envelope & stamp will cost over £1). If you sell your balloons for £50 each you will probably get a-lot of free entries.

More on legal requirements: balloonrace.uk/2020/11/1…