USPS First Class Mail Shipping Rates Differ by Item - ?

[therustygardener]

Maybe I'm imagining things but when the new rates first took effect this week, I hand wrote rates down (actual & discount) for each oz. so I could familiarize myself with the changes. That was January 28. 

I'm noticing that the rate is not consistent with the type of item. For example, I'll sell a sewing pattern and it's one amount for 5 oz and then I'll sell a pocket mirror for 5 oz and it's a different amount.

Also, on January 28, when I sold an item, the rates were $4.69 (actual) and $3.49 (discounted) price for 5 oz. On February 1, they are $4.53 (actual) and $3.28 (discounted) price for 5 oz.

Tonight, I notice that a 15 oz USPS First Class (in Pocket Mirror category) is coming up as $8.00 (actual) rate. On Monday, January 28, I had it written down as $8.60.

It used to be a straight rate for 1-4 oz, etc no matter what the item was as long as it was USPS First Class. Now it seems to be different depending on what category the item is. 99% of my items are smalls and are USPS First Class. Except for USPS Media Mail which is by the pound, it seems difficult for me to get a handle on what I should be charging for my shipping. I don't want to gouge a customer for a higher than customary flat rate either.

Has anyone else noticed this?