Ultraverse 20x4.0, 24x4.0, 26x4.0, 20x3.0, 26x3.0, 16x3.0, Inner Tube Set with Schrader Valve - Ideal for Mountain Bikes & E-Bikes with Fat Tires - Durable & Reliable 2 Pack with Tire Levers

What does on mass mean? What country is this? I would imagine a 20"x3" (equal to 508mmx76mm) inner tube shouldn't be that hard to source? In the US Amazon sells a bunch of varieties, like this for example: https://www.amazon.com/Ultraverse-Inner-Schrader-Valve-Bikes/dp/B0DLTL4RKS I guess if it's truly difficult to source, I'd more likely put in tire sealant preemptively. You can still use a good old tire patching kit as well. However it does seem a bit more involved to remove the wheels on these cargo ebikes, so I think it's best to use a sealant either preemptively, or have some ready on the first flat. Did you already put sealant in your tube?