The error does not reproduce. Check the correct of your settings of plugin "Fixed or By Weight and By Total", they are calculated by the formula: [additional fixed cost] + ([order total weight] - [lower weight limit]) * [rate per weight unit] + [order subtotal] * [charge percentage]
Please describe your actions as an example of one of the product, provide the settings to use on the product detail page. Describe in detail the settings of the plugin "Fixed or By Weight and By Total". So you can follow your steps to reproduce your path.
Method: 1st Class Recorded Order weight from: 0.00 Order weight to: 600.00 Additional fixed cost: 4.40 Rate per weight unit: 0.00 Lower weight limit: 0.00 Charge percentage (of subtotal): 0.00
Method: 1st Class Recorded Order weight from: 601.00 Order weight to: 1000.00 Additional fixed cost: 6.50 Rate per weight unit: 0.00 Lower weight limit: 0.00 Charge percentage (of subtotal): 0.00
Method: 1st Class Recorded Order weight from: 0.00 Order weight to: 600.00 Additional fixed cost: 4.40 Rate per weight unit: 0.00 Lower weight limit: 0.00 Charge percentage (of subtotal): 0.00
Method: 1st Class Recorded Order weight from: 601.00 Order weight to: 1000.00 Additional fixed cost: 6.50 Rate per weight unit: 0.00 Lower weight limit: 0.00 Charge percentage (of subtotal): 0.00
Didn't you define a range above 1000.0? In your example weight was 1300. You should add other weight ranges, if there are more prices and end the last one with high Order weight to: 10000000 For sanity check create a test product, without attributes, but with a weight of 1300 and see results