When I first started out shopping by bike, I pretty much stuffed my backpack full. I had an MTB so a pannier wasn't really an option. If things ended up not fitting, my handlebars came to the rescue to hold the rest of the bags.
With my current bike however, things are a little different. The rear pannier rack enables a few more options and things to try...
Carabiner
Tried and tested, never failed me yet. Using the carabiner I can carry a pretty heavy bag on the rear pannier, without shelling out $$ for an expensive cycling bag.
The bag does stick out a bit depending on how packed it is, which is something to be aware of.
Motorcycle helmet clips
These mainly allow me to secure boxes and water bottles to the top of the pannier rack, while still being able to quickly attach and detach shopping bags from the side.
While the helmet clips themselves have been flawless, the bags themselves have been less than stellar - every single one of them have ripped so far ☹️
DIY pannier bag
I did consider getting a proper cycling rack bag... but they're a bit costly and can't really be used for much else.
After some research I made my own, using a reusable 'cooler' shopping bag from a supermarket, and a cheap pannier attachment kit. I had some short bolts, nyloc nuts and washers available to put it together.
It's so much more convenient to use than the other two methods above, but I haven't put it to the test yet with heavy shopping. The zip is a pretty nice touch to keep shopping cool and dry.
Additional pictures
How do you guys carry your shopping, or any goods in general?
I've got a rear rack with two metal collapsible baskets on it, one per side. They both fit a bag nicely and can carry significant weight. I shop with two bags, or if I only need one, I move some stuff over to the other basket to keep the weight more balanced over the rear wheel.
If I need to carry more than two bags, I'll hang a third or fourth from the handlebars, again ensuring balance as best as possible.
Bad memories for me here. This was my first arrangement when I started commuting. I brushed my knee against the basket and it ripped open a huge gash. I moved on to Ortlieb panniers that held more volume, were waterproof, and didn't leave me bleeding out on the sidewalk.
A rear or front basket? I can't stand having one on the front because it makes the bike way less stable (which is why bags on the handlebars is a last resort). Having one on either side of the back wheel is much better for weight distribution.
These were Wald folding baskets on the rear rack, when panniers would normally go.
Hm. I guess it depends on the distance between the basket and the crank, and probably the individual using it. On mine, I can't hit the basket with my knee even if I try.
When mounting or dismounting and swinging your leg over the back.