E Commerce Sales as Part of Total Retail Sales

How much of the total retail sales (as a percentage) in the United States is done online?

2019 Total Retail Sales (in millions of dollars): $5,452,351
2019 E-Commerce Sales (in millions of dollars): $595,916

This means about 11% of 2019 sales were done online. Source

What is the current growth rate of online sales in the United States? (also in a percentage)

In 2018, conusmers spend $523.64 billion, in 2019 consumers spent $601.75 billion. This means that Ecommerce grew by 14.9% from 2018-2019. Source

How does that growth rate compare to previous years?

Using the same source as above, online sales from 2017, grew 13.6%, up from 11.8% in 2016, and up from 10.7% in 2015. In a rough average, online sales has increased by 0.8% - 1.6% from year to year. Source

Is the growth rate expected to decline, slow, continue or speed up in the future?

The growth rate is expected to continue to grow at an exponential rate. And in my opinion, an even more increased one than projections due to COVID-19. Source