Honestly, the new multi-function shoe on the R10 is a pain. I had issues with my older Canon flashes misfiring constantly too because the pin alignment is slightly off. Since you have a wedding next weekend, you cant risk it. I ended up switching to Godox and it has been way more reliable. Here are the best options under $300:

• Godox V1-C TTL Round Head Flash for Canon
• Runs about $260, has a great lithium battery that lasts all day, and recycles incredibly fast.
• Godox V860III-C TTL Flash for Canon
• Around $230, basically the same performance but with a traditional fresnel head. If you want to try saving your old flash, you need the Canon AD-E1 Multi-Function Shoe Adapter to seal it properly, but adding another piece just introduces another point of failure. I would just grab the V1-C.

Oh man, wedding photography is a high-wire act and gear failure is my absolute worst nightmare! I had an old flash misfire right during the first kiss at a ceremony last year, and I still get anxiety thinking about it. Now I always carry backups and test connections. Quick tip for you: always wipe the R10 hotshoe contacts with a microfiber cloth before mounting. Sometimes tiny debris on those new multi-function pins causes the misfires. Before we look at specific options under $300, are you planning to shoot TTL all night, or do you mostly stick to manual? Knowing if you need high-speed sync for outdoor shots helps too! I absolutely love the Godox V860III-C TTL Li-Ion Flash for Canon because the lithium battery is a lifesaver for long days, but let me know your shooting style first!

> Shot events for years but my old Speedlite 430EX II is acting up on the R10 new shoe, constantly misfiring. Jumping in here because I ran into this exact same problem and it's super frustrating. Unfortunately, the issue is that older Canon flashes have a thick rubber dust sealing gasket at the base. On the R10's multi-function shoe, that rubber boot prevents the flash from sliding all the way forward, so the pins dont line up properly. It's not as good as expected from Canon. For your wedding next weekend, you might want to look at the Canon Speedlite 430EX III-RT. It has a redesigned mounting foot that actually fits the newer shoe without getting stuck on the weather sealing. Plus it's under your budget. If you need help setting up the wireless RT system for the wedding, let me know!