<p>I have designed a cupholder for my center console in my truck with PETG. I was hoping it wouldn't deform and it didn't for a while but we got some high temperatures the past few days (around 32-38 °C or 90-100 °F).</p>
<p>Kind of figured it wouldn't hold up mid-summer but I was wondering if ABS would be my next best choice or if there is something better for this. It does see some sunlight but not a lot.</p>
<p>I do want to start producing some to sell and don't want it deforming in high-temperature areas. Also, I have an Ender 3 Pro so I can't do super crazy filaments.</p>

ABS, or preferably ASA which is "a better ABS", is probably your main option. ASA holds up better under UV/sunlight and is easier to print (less warping). Like ABS it should be printed with ventilation, and benefits some from an enclosure but can be done without it or with a primitive one (cardboard box).
Another great option would be TPU. It's not subject to a glass transition temperature above room temperature (my understanding is that technically its
$T_g$
is very very cold, but that may be a misunderstanding) and does not really warp/deform permanently until you reach temperatures near what you could print it at. I've used it as a mold for melting crayons in an oven at 175 ˚F (80 ˚C) with no problem. Depending on your perspective it could be easier or harder than ABS to print. If your extruder is bad at handling flex materials you might have to go really slow, or you might have trouble with jamming on retraction, but unlike ABS it has no heat/warping issues while printing and can be done even on a cold bed, and does not particularly need ventilation (although as always, be careful if you have pet birds around).
Note that while TPU is considered a "flex" material, it can be fairly rigid at high infill with rigid infill patterns like triangles or cubic, or printed 100% solid, especially if you go with harder variants. 95A is typically the highest you see but sometimes you can find 98A.