The bi-plane stamp is in the scott stamp catalogue, it is a dominican republic "special delivery" stamp from 1920, the catalog number is E1.
The red stamp is a dominican republic regular issue from the 1920's. There are a few different versions, so I'm not sure which one it is.
Both of these stamps seem authentic, but I'm not an expert. Their value is only a few dollars US.
The real value is in the "cover" and the cancellations. I don't collect flight covers, but there are many people who do. I've sold some first flight covers before on eBay and they brought in a fair amount of money.
Perhaps you were asking about the cancellations instead of the actual stamps? It appears that there are some blue cancel's of this cover on eBay, are your cancels done in black ink? There should be a catalog of flight covers that you can reference. There are a ton of fakes for sale on eBay, so, no telling. Your's might be the authentic one, or perhaps the person doing the covers swapped ink at some point. (unlikely

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1928 Dominican Republic First Flight Cover to Cuba Lindberg Special | eBay
It would be pretty easy for someone to take a $3 worth of stamps, but them on an old envelope, stamp them, then sell it for $70+