Wherever you shop, whatever style, make sure it FITS WELL. Mens shirt sizes have changed! A shirt that used to be labelled Large is probably labeled Medium today. Really! My father buys larges and they shoulder seams are down on his arms and there’s extra fabric in the body. Shirts should fit, not hang and droop, like many too-large polos do today. Look for the right fit, not the right letter (S,M,L, XL) on the label.
If you MUST shop mail order, budget money for return shipping. You might save yourself time by ordering two sizes.
Of course, you could shop locally. What’s more of a hassle: using a changing room for 4 minutes, or repacking & reshipping the wrong size back, then waiting for the right size to arrive?